We had the same issue. Your only choice is to boot them if they are unresponsive and continue to break the rules. If you don't, then other guild members get jelly and start attacking too, and before you know it there are no rules and it's a FFA.
It's unfortunate that not everyone can let it go, because really who cares if they did 5k damage before it was time? That's probably all the damage they can do period and they just wanted to get it out of the way. Their attack makes no difference, everybody just relax. But no, people get knots in their knickers and figure they can do as they please since the other guy did too. And then everything falls apart.
You have to boot them.