Listen, first things first about befriending people, online or IRL, don’t let them prey on you or manipulate you. Not everyone on online is someone you wanting to trust, especially in fandoms. Things like grooming, harassment, death threats, and everything in between is just an awful thing and if you’re a minor in this site, please be wary of who you trust.
I agree it’s a good idea to be selective about trust, but predation and manipulation aren’t about “letting” someone do it to you. Very often one’s ability to be skeptical, to set boundaries, and to leave are systematically dismantled until it takes a lot of strength and perspective to be able to bust out. It’s particularly hard for minors when dealing with someone older.
I’d say if something in a relationship feels wrong to you it’s a good idea to talk to someone trustworthy and compassionate. Unfortunately predators very often instill shame and guilt in their victims, making it hard to speak out. For that reason I think it’s important for communities to put out warnings, public and private, about who has been proven untrustworthy so that people, especially minors, know who to stay away from. It’s much easier to steer clear of potentially toxic situations than to try to get out of one.
That doesn’t mean the predatory person has to like die or be shunned forever or anything. I believe in personal growth and people learning to be better. They can work on themselves away from minors and anyone else who could be vulnerable, however, something they would accept if they’re truly sorry. It takes a lot of hard work to change one’s values, which are at the heart of abusive behavior. I would far prefer someone who has preyed on people to suffer social seclusion than to gamble a minor’s well-being on the chance of the wrongdoer being all better now.