Why not just ban them? Funny thing is the term 'troll' is subjective. It could simply mean that they don't like or agree with you. And to you that makes them a troll. And on the flip side they might feel that *YOU* are the troll.
It's a silly word that is overused by members who are inept at dealing with strong minded, colorful members. Administring a forum isn't for everyone.
I usually just ignored them publicly, send them a PM asking whats wrong, try to talk it out and figure out whats making them want to act out. Sometimes that works, sometimes people just want to feel seen/heard and then they feel better.
Every Site has options to ignore or ban those one is not comfortable with. One cannot get along with some although they may not be doing anything wrong
It seems like trolls have moved to social media. Of course, there not really trolls, they're just regulars who are bullies. Anyway, it would be difficult to get rid of them cause, well, they're regular and perhaps most people on the social media agree with them.
Anyway, when it comes to forums, assuming they ever got really big which is hard these days, you would see some trolls and the same idea that applies to social media also applies here. I would say disciplining them (or banning them) would be a good idea, but like I said, bullies are often popular.