The chance of being on the internet for an extended period and not running into a troll is 0%.
I can tell you as a former forum admin, that there is no way to avoid these people showing up. The rules don't matter, banning them doesn't matter, being a calm and relaxing place won't matter either.
What did matter is that the rules made sense and were followed indiscriminately by the mods and admins. It also helped to have a forum dedicated to spam and allowed for breaking of many rules (racism/harrassment and CP being excluded from the exception)
My banlist over a million users was 20 people long. I don't consider this sum to be hard to handle and at 1.2m registered users (without a need to register to read), I think maybe the problem is actually in the rules/application.
People hate feeling like they are being treated unfairly.
We would also enforce the rules when racism was targeting white people.
Because they radicalised each other and then went out into other subs no longer caring to hide their bigotry and racism.
If you you live in an apartment on a street with a nightclub on it, you're going to have drunk jerks on your street at night.