ScarletRose: I think the only fair way would be something that if you put person A on your block list, and both you and A sign up for a tournament, you would be automatically withdrawn. Otherwise, it's too easily abused.

OTOH, I wouldn't mind if tournament creators could have a list of people that are prohibit from signing on. Currently, one can already remove signed up players, but that's an almost impossible task if you have many tournaments.