This is inevitably going to be a heavily context and detail-dependent affair. Since you don't seem to know to any specificity what the individual actually did, who the victim was, or how recent this was, you're going to struggle to convince others of the importance.

Thusly, I'm not sure anyone could give good advice in this situation, other than to talk to those previously affected and what their actual views on the perpetrator are - otherwise anything you do is going to be based on complete guesses. Sometimes that's all you have to go off, and in those cases, you can only instead ask people to err on the cautious side.

You should probably try to find out what actually happened

Have you tried punching him in the face? Most people don't like that and if you do it enough they'll usually just go away forever.

But for real if you don't know details about this then you probably shouldn't try to push the point. Either get more info so you can be persuasive or wait until the dude does something fucked up again (not ideal).

But also, a lot of abusive people will just lie and wiggle out of any accountability so you might end up having to punch them in the face anyway. In my experience you usually have to be very aggressive with them at minimum or they'll brush it off and act like you're overreacting, joking, etc. I've confronted people like this several times over the years and every single time it's been a hassle, they never just accept it and move on and they definitely don't apologize

Good luck trying, I've found no success whatsoever. I've had to move on from the scene altogether

Raise it with the group, try to get their response recorded, then forward that response to the orgs/spaces this person was banned from.

Social pressure works. We had that happen in our local DSA and a member who was on SC was called out for inappropriate behavior by another org and they immediately removed them from SC and did an investigation, then polled the membership (mostly just to notify them and find out who would be factional about it) then kicked them.

like asking how to get a fish out of water