Anyone who makes policy or takes punitive action without fact-checking things, or basing decisions on social media rumors and the mere existence of accusations, etc. is not helping themselves nor anyone else. The way to deal with any such event as the OP is the same as it always has been: Verify what actually happened to the best of your abilities; take appropriate action based on that; tell social media and rumormongers to take a hike. And then if you want to make a policy about such things just be sure you have means and the will to enforce it, or again, you're not helping anyone.
Geez, I'd have thought it was S.O.P. to have any DM of
any game at any con of reasonable size sign some form that
spells out what kind of subject matter is okay or not okay. Don't introduce questionable material if you're a DM, don't permit it being introduced by players at your table. If you have no such policy in place then slap
yourselves on the wrist and PUT IT in place. If anyone was out of line, official policy or no, take appropriate action and move onward and upward. If someone wants to run a dark, deviant, questionable, or downright offensive anything at your con
you don't have to let them (unless if that's the whole point of your con?) and if you do it's your own flippin fault if people take issue with it.
This isn't hard to figure out. When people do offensive things just kick them out and tell them not to come back. If an incident is blown out of proportion then blow it back into proportion. It's not magic. Just be decent, reasonable humans - both to avoid causing problems and to solve them when they arise. Is practical decency really so far gone that people don't just grok this by reflex?