Bad-Spirited Roleplaying:
The RP Sin That Needs To Be Against The Rules Already

Rendering other people's characters unable to meaningfully participate in the game. This can include transformations that render other people's characters unable to contribute anything to the plot (for example, de-aging them into very young children or babies, or turning them into animals). It can also include instantly and easily tying them up or imprisoning them with absolutely no opportunity for them to dodge or try to evade capture. It can also include having interesting and important things take place somewhere the PCs can't get to, or by literally locking them outside of where everything interesting is going down.

Might look like this, but isn't: A character being locked up as a consequence of being antagonistic to other characters, or as a consequence for illegal action, where the player could reasonably expect it as a consequence. It's not bad-spirited roleplay to put the Starfleet officer who just tried to assassinate the captain into the brig, for example. Nor is it bad-spirited roleplay to bench the "superhero" whose lack of self-control constantly creates problems.

Willfully harming or risking the welfare of the PCs. This can include using weapons, spells, or powers in circumstances where the other PCs are likely to be harmed as collateral damage. It can also include having their characters throw power tantrums, or just playing characters who put off dangerous power surges whenever they get upset... and having them get upset very easily.

Might look like this, but isn't: A character is deliberately played as an antagonist, and is played in such a way that the PCs have a fair chance of victory.

Glory hounding. This can include people having their characters always come up with the solution or fix for any problem regardless of how much sense it makes (EG, you wouldn't expect a roboticist to be able to whip out a cure for an alien virus), or not allowing anyone else to meaningfully contribute to the solution (such as by declaring everyone else's efforts to be failures). It can include always declaring that their characters know exactly what's going on and exactly what needs to be done, or declaring that their characters are essential (or even central) to solving the current crisis and treating everyone else's characters like supporting characters or sidekicks.

Using the game as a tool to annoy or bully the players. This can include snatching away characters' food or personal belongings, not allowing them to partake in something that should be available to everyone (EG, drinking up all the coffee so no one else can have any), or by threatening or actually using violence against the other PCs. It can also include insulting or belittling the PCs and their accomplishments, disregarding their opinions or never giving them the chance to speak up in the first place, or by pulling rank. While it's important to remember that characters are not usually their players, some players deliberately exploit this to bully others with plausible deniability.

Might look like this, but isn't: A character is deliberately played as annoying and bothersome, but the character is intended to be perceived this way, and the player has no trouble accepting reasonable consequences for the character, or is happy to dial the character's behavior back if it becomes too irritating.

Endlessly whiffing other PCs' actions against them. Ever run into those players who spend upward of half an hour declaring that every hit taken against their characters is a miss, thus preventing the current scenario from resolving and keeping the plot from moving forward? Yep, this is that. It's extremely unfair and selfish to force the game to a halt like this.

Making everything all about their characters all the time. This can include making their characters constantly require attention and care from other PCs, constantly putting their characters at the center of whatever plot ideas they come up with, or putting down other characters' hardships and struggles to talk about how much worse theirs are. They may even have something bad happen to their characters whenever they feel like people aren't paying enough attention to them or aren't showering them with enough pity or forgiveness. (See also How To Spot & Handle Parasitic Roleplayers.)

Imposing their own ideas onto other players' characters. For example, a player creates a character who is supposed to have no interest in sex whatsoever. Then another player comes along and decides that this person's character is secretly sexually repressed and desires "liberation," and goes along insisting that this character just needs a dose of good loving to see what's missing. Or a player decides to reinterpret a mostly-villainous character as an anti-hero, but another player continues to treat this person's character as a straight-up villain, despite this character not having done anything antagonistic toward any of the PCs in the actual game. Or someone might decide to play a character with PTSD and says as much, but another player decides that the character is simply "wallowing in self-pity," "trying to play the victim card," and "needs to get over it" when this character ends up suffering a traumatic flashback sparked by the other player's character.

Might look like this, but isn't: Characters being wary around another character who has canonically done a lot of awful things and hasn't had time to establish a pattern of improved behavior, or players refusing to play along as if a character has a certain trait (EG, courage, kindness, wisdom, etc.) when that trait never actually carries into the character's behavior.

Playing characters who readily blow up or fall apart without any warning. Essentially, playing characters who come completely unglued over slight provocations, whether that means flying into a violent rage, collapsing into a major fit of angst, or something along those lines. This can be a form of game sabotage, as it usually disrupts the game, either significantly delaying or completely halting the plot. Furthermore, it can put players on edge by making them constantly worried that something they'll say or do will set the character off and derail the game, and that's not a mood anyone should be roleplaying in.

Being pointlessly destructive or disruptive. For example, starting a fire or setting an explosion in the PCs' hangout, interrupting an ongoing plotline by making a random villain attack everyone, attacking other PCs for no real reason, breaking or hiding equipment they need, and similar things. This can sometimes happen because players are simply bored and don't know what to do, so it's a good idea to try talking to them first if they start doing this. If they persist despite all attempts to get them engaged, however, they probably shouldn't be your game.

Being completely self-absorbed. Self-absorbed players rarely have their characters truly interact with others, instead spending their time writing about their characters' solo antics or detailing the depths of their mental anguish. Because they're not interacting with anyone else, they're failing to contribute anything of value to the game, which means that there's no point in keeping them around. If they're that bound and determined to fly solo, they can leave the game and write fanfic.