Something I've never solved in 18 years. It tends to be one of these though,
Set up an alternative member group which has a limit on the amount of posts they can make in a day, I set this as 10. This is an attempt to stop the troll ruining multiple topics with lots of posts and creating a nightmare for me to sort out.
If that doesn't work then post restrictions for a few day or a week. Sometimes this just makes them worse, so end up banning them and they rejoin under a new username and the process continues. 🤨
What would be perfect though, and I've requested this for so long is to be able to ban a member from a topic. This would help so much in these situations.