Your user.txt file has nothing to do with bans. It only controls group access.
ULX has no command to ban by IP address. I can change my IP address within minutes. Most trouble makers know this.
SteamID works best because it is less likely to change.
I've just done a quick search on google, and on the Valve developer wiki, and am not even sure the newest Source engine allows banning by IP (I can't find the command, but, I only looked about 5 minutes)