I had a campaign end that way, It was incredibly sad and anti climactic. Now when people do that I just say no. Don't go for punishment, just refuse to recognise the attack.
I was a player in a campaign where two of the other players just full-on sprinted past every sign post the (Pathfinder) GM threw up saying, "This character is here to give quests. He is much higher level than you. Don't attack him. He will slaughter you."
They pulled out their weapons and made to attack the guy and my CN character was like, "You have fun with that. I don't want to die today." The quest giver and our other party member tried to talk them out of it for a couple rounds AFTER they started attacking (and doing no damage, naturally). The NPC then started using non-lethal spells to try to defuse the situation. A couple rounds of that and the GM finally gave up went hostile. Two or three AoE attacks later and both characters were gone.
They rolled new characters, but I didn't last much longer in that campaign.
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.
I had a campaign end that way, It was incredibly sad and anti climactic. Now when people do that I just say no. Don't go for punishment, just refuse to recognise the attack.
I was a player in a campaign where two of the other players just full-on sprinted past every sign post the (Pathfinder) GM threw up saying, "This character is here to give quests. He is much higher level than you. Don't attack him. He will slaughter you."
They pulled out their weapons and made to attack the guy and my CN character was like, "You have fun with that. I don't want to die today." The quest giver and our other party member tried to talk them out of it for a couple rounds AFTER they started attacking (and doing no damage, naturally). The NPC then started using non-lethal spells to try to defuse the situation. A couple rounds of that and the GM finally gave up went hostile. Two or three AoE attacks later and both characters were gone.
They rolled new characters, but I didn't last much longer in that campaign.
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.