Neither option worked. Reinstall or quotation marks.
Reinstall of what, gmod?
As for quotation marks, they should be just like the other settings in your server.cfg
I'm going to say its either DarkRP, ULX, or something just not working right.
Would reinstalling ULX or getting the SVN ULX fix this?.
You were having the problem _before_ you installed ULX according to your Facepunch forum thread.
In fact many ULX versions ago we noticed that Garry didn't put the required 'load banlist' (exec banned_users.cfg) commands in server.cfg, and we were getting (false) blame for clearing the ban list when ULX was banned. We worked around that by loading the ban list at ULX startup.
To me, that rules out ULX. No, SVN won't fix your issue. Fixes several other quirks and oddities, but none related to the exploitation of a server.
If that were the case, we would have released a new update long before now.
As for other ideas; Sorry, I personally have no exact 'this is it' at this point.
Possibilities.
DarkRP (have you tried running just a plain build server/Sandbox?)
Lua Exploit (Difficult to find... running a build server from a straight brand spanking new no addons added Gmod server install would be the only way to truly test)
I saw you mention kickid somewhere, are you also using 'banid' and 'writeid' after kicking them? (Kickid doesn't ban I'm pretty sure)
Someone may have your rcon pass and they unban themselves?
Another admin on the inside unbanning others?
(A) Version(s?) of DarkRP (I don't remember which) would allow someone who wasn't an admin to use admin commands through someone who was an admin.After banning someone, does the STEAMid of the player you just banned and kicked appear in 'listid'.
If it does, and they get in, you have a problem with Gmod server code, or a dll/binary plugin that avoids the ban portion of the server engine.