Keep in mind Method 1 of the link JamminR posted requires the admin to be on the server at the time.
There is no Console way to remove a user from the ULX generated users file while they are not in the game.
On a further note... you can remove them from teh file manually (method 2 from the link) however since the file loads only once per map change, if the admin should happen to join after you remove them, but before the map changes/server restarts they will still have their accesses as an admin.