If you're not going to remove donator group access from them (a PRIVILEGE imo, not a RIGHT), then you'll need to deny him the specific commands from him that you don't want him to use.
I'm sorry, but I don't know how to tell you to do this in XGUI (It's probably easy, I just haven't loaded Gmod in a while to look)
o ulx userdeny <player> <command> [<remove explicit allow or deny instead of outright denying: 0/1>] - Remove from a user's access.
So it would be
ulx userdeny "Your players name" "ulx command1_you_dont_want_him_using"
ulx userdeny "Your players name" "ulx command2_you_dont_want_him_using"
If you later want to grant the command, you add a 1 on the end using same command.
ulx userdeny "Your players name" "ulx command2_you_dont_want_him_using" 1