The cleanup commands in Gmod have nothing to do with ULX. ULX has no cleanup commands (Because Gmod includes them already)
If you are admin and using game client GUI, there are buttons within the game menu (usually Q button) under "Utilities" tab.
If you aren't using a game, but are instead running dedicated server;
From console;
To clean up the entire server type "gmod_admin_cleanup" in console.
To cleanup a certain object, use "gmod_admin_cleanup <object name>"