ULX stores additional ban information in an _additional_ file bans.txt. If that's all your deleting, its not enough.
(We store reason, temp bans, and admin who_done_it)
That's NOT the standard Gmod file that also stores bans, and gets executed every ULX ban change and at server start up.
Though you may have other admin mods or addons conflicting, ALL bans with ULX are still also stored in Gmod's (defaults to) /cfg/banned_user.cfg or banned_ip.cfg
That file gets loaded at startup.
See our FAQ.
How do I unban someone?