Make it clear that PCs have nothing to gain by attacking. If they still try I try to talk them out of it.
If they try to attack, say no and ignore them
If for some reason, you have to recognize their attacks, just make all of them miss. Don't be descriptive, just say "miss" and have the NPC sit there. This is not a fun way of playing DnD at all, the point is to bore them out of it. Never attack, that's going to mess up the campaign for months.
Consider not inviting the player back to the next sessions. Problem players ruin campaigns and it's usually because they'd rather do something else. Don't force them to keep playing, let them go and your campaign will improve.
It's important to avoid punishing the entire party because of one problem player. Your DM's mistake was giving the problem players chaos which is what they want. That and choosing their enjoyment above players who are making the game more fun, not less.
My procedure is as follows:
It's important to avoid punishing the entire party because of one problem player. Your DM's mistake was giving the problem players chaos which is what they want. That and choosing their enjoyment above players who are making the game more fun, not less.