This causes a lot of problems for the con and I don't think it's worth their effort. They'll have to make sure convention personnel are aware of who this person is, what he looks like, and keep an eye on him at all times to make sure he isn't violating the conditions of his attendance. Most con personnel (and GMs who aren't even con personnel) are volunteers and they shouldn't be burdened with this sort of task. And then there's the liability issue the convention may have to deal with. If there were an incident and the convention was sued they might be asked by a solicitor why they allowed someone they believed to be dangerous to attend. And of course they'd also take a publicity hit if they allowed this individual to attend and he repeated his bad behavior.
I'm not keen on destroying this guys life either. But if I were running a convention I just wouldn't be willing to jump through hoops to ensure this guy could participate.
And, really, that's what this comes down to. Every time these sorts of things come up, everyone jumps up and down talking about "investigating claims" and various other things that "should be done". It's just not feasible. It's no different than a bar or any other social gathering area. When a complaint comes in, you deal with it and move on. As a con runner, you don't have the right to "question witnesses" in any sort of legal sense, nor is it your responsibility to do so. I don't want some con planner sitting in judgement over "well, I guess that complaint is good enough, but, that other one isn't." No, it's not how this works.
A complaint is made, and it gets dealt with. 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? If you do, then fair enough, be up front with con goers - tell them that you are not policing GM's and GM's are free to present whatever game they want, and caveat emptor.
OTOH, if you are billing yourself as an all ages convention where you KNOW that there will be minors present, then it's your responsibility to ensure that games fit within those parameters and if a game or GM doesn't, they get the boot. All the crap about "due dilligence" and "investigate" and "innocent until proven guilty" doesn't apply. Con runners have neither the time, nor the inclination to "investigate". You treat a claim as valid and deal with it.
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. 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):
Now in its 13th Year, UK Games Expo (UKGE) is the largest Hobby Games Convention in the UK. A fun event appealing to families and the general public as well as the enthusiast.