Even the relatively lax enforcement of anti-competitive laws we do have is likely to prevent that particular scenario.
I'm not sure it is possible to write a good law that says you can't act against people for some set of reasons, without impacting the ability of venues to deny service in more reasonable cases. (Current trespassing law pretty much relies on notification, if you are told you aren't welcome in a private place and then refuse to leave or later re-enter it, you are trespassing).
Like say an attendant observes that someone is visibly intoxicated, sees them vomit in a spot that isn't part of the venue, and then tries to deny them service. In that case, an escape hatch that lets them add people to a list if they are ejected wouldn't work, but it's perfectly reasonable to deny that person entry.
The better mechanism to deal with things like this would be social pressure. Arbitrary denial of service is outrageous and people should punish them by not doing any business with them (don't book events there, don't attend events there). But no way is anyone going to inconvenience themselves over a few lawyers getting a bad deal. Maybe we'll get a more charismatic scenario sometime in the future.