It's not really worth a ban in my opinion. Truth be told Zwieb has mentioned multiple times its in the game for rpg reasons so if someone wants to be an asshole and kill the team during a boss fight, they can. Some people would find that funny, most would not.
So it comes down to really just not playing with people who harass, and if they continue to do it in a way that could affect the community or discourage new players to continue playing then sure, it is indeed ban worthy but if they pvp you once to piss you off it's more personal than bannable.
Harassment and repeated offences should lead to bans as opposed to one singular dick move.