Healers who rescue pull tanks into mobs.

[Veis_Alveare]

(slightly controversial) option since most of the sensible "just literally say something in chat like 'hey i'm pulling big' or 'i'm undergeared/new to this dungeon so i'll start off small'" suggestions have already been made:
if the healer or dps are running off into mobs to either drag the back to you or rescue you to them?
let them die. then say "i'm undergeared/new to this dungeon so i'll start off small" in chat.
you don't actually have to pick the mobs up off of them that they went out of their way to aggro. yeah, if you're in proximity after they die they'll attack/kill you, but at the point has hopefully been made by then.

of course, again, you could also just literally say anything to the rest of the team.

Refusing to participate is actionable ingame, should you actually try this accept the possibility of winding up in GM jail.

[Silica-chan]

Refusing to participate is actionable ingame, should you actually try this accept the possibility of winding up in GM jail.

The tank not picking up mobs that were aggroed by someone else on purpose won't get anyone in trouble. Why would it? If you willingly walk into enemys it's your fault for dying in the process. It's just a matter of "difference in playstyle" or a "bad" tank. The vote kick feature exists to deal with stuff like that, GMs won't act unless you start insulting each ohter in the chat.