No, it's not how this works.
I would hope that if you are going to ban someone indefinitely, which could possibly lead to bans at other conventions you at least take the time to listen to both sides and gather what evidence you can. Sure cancel their games for the convention while you do the investigation, because you need to act and you don't have the time.
Even if we want to take the GM at his word and it's "only" kidnapping children and covering them with feces, well, as a con rep, do you really want that representing what you think should be played at your con?
No children were involved. The game was advertised as over 18, the players were all over 18, and the characters were all over 18. Stop trying to make it sound worse than it was. The con might be family friendly but they clearly allow 18+ limitations as his wasn't the only game with this restriction.
It utterly baffles me how much slack people are willing to cut folks. I mean, there's nothing in Things from the Flood or Tales from the Loop that would even hint that this would be an expected scenario.
Perhaps not, but people use system to run there own scenarios not necessarily close to the setting, I've not been able to see the full blurb, but if "lad's holiday, Club 18-30" have a reputation which certainly includes blackouts and not remembering how you ended up in a horrific situations.
A thinly veiled reference to a gang rape scenario isn't what folks signed up for, I'm thinking, nor is it even remotely acceptable at (From the front page of the UKGE):
Again it wasn't a gang rape scenario, at worse the rape (if it occurred at all) happened off camera (veils are a pretty standard technique in RPGs) so it wasn't described, and rather than a scenario is was a few minutes, within the several hours of the game scenario.You keep using phrases to make it sound like it was a 3 hour ordeal of constant child rape, which anyone would agree would not be suitable. One off-camera ambiguous event during a long session in game labelled 18+ horror is not the same thing at all.