Griefing and toxicity
Also said as Griefer, Griefing, Toxicity, Trolling, Harassment, Toxic players
Griefing is using the game's own rules to spoil it for others, such as blocking a door or killing a teammate. Toxicity is abuse aimed at other people, usually in chat.
Both drive players away, and the people who leave are the ones you most want to keep. Reports and bans come after the harm, while design can remove the means of doing it in the first place.
Say it
“Team members pass through each other, and friendly fire does no damage. Nothing a teammate can do should be able to stop your progress. Then report and mute are the second line of defence, not the first.”
Where you’ve seen it
Journey's players cannot hinder each other at all. EverQuest let players fight only if they had switched PvP on. League of Legends, long known for toxicity, tried a Tribunal in which players reviewed each other's reports.
Not to be confused with cheating
A griefer is often playing within the rules, and that is what makes it hard to punish. A cheater is breaking them.