If you talk to him and he still tries to ruin everything, then kick him out of the group.
I've been in two groups that had to kick someone out for being disruptive in every game. In one of them, the guy asked to rejoin a few weeks later and actually wizened up, the other guy I still see at work sometimes, but never really liked him in the first place.
As far as invincatroll (if he does try that).
Two snipers with high powered rifles. Big tank is going to attract a ton of attention. And since the range of a sniper rifle can be over a Kilometer, he can't very well return fire.