One thing that I just had to deal with today was someone holding the elevator hostage unless people gave him crystal energy. Furthermore, he was friends with the leader so at the very end of the level they just kicked everyone who refused. Therefore I think that just like one can set friends, one should be able to set enemies and this way, whenever you do find party on the elevator, you will never have to see that person ever again.
As for what happens if he tries to join a party and your not the leader, I say, that if half the current party or more are enemies with this person, he can't join.
Lockdown however will probably have to be unaffected by making someone an enemy simply because that might be abused
I would prefer if there were just better "report" options for this type of activity so these people could be banned so nobody has to deal with them.