I'll look at server plugin banning. I can autodetect ulx, SM will need a convar setting.
edit: Stuff for this is now in the SVN, will be in the next gmod update (maybe tuesday).
Edit: I spoke to some ULX staff and all the signs point away from it being a ULX ban, please help? :D
I believe gmod uses Source's standard set of "banid"/"removeid"/"listid" commands. So if you run the command "listid" on the server, the ban should be in there. Then you can remove it using "removeid <steamID>" (or if you know the steamid you could just do that right away).
If nothing else works, then rebooting the server should reset these bans.