Method One (NEW as of ULX v3.50)
Open XGUI and click on the "Groups" tab. (Shortcut: "xgui show groups", chat: "!xgui groups"). In the topmost dropdown box, select the group which the user belongs to. Select the user in the list of users within that group, then click "Change->Remove User".
Method Two
In server console or using rcon from game (default), type "ulx removeuser <name>".
Method Three
Open <gmod root>/data/Ulib/users.txt in notepad or another text editor, and remove the block that will look like this:
"(friend's name)"
{
(data in here, multiple lines)
}
[Edit-Stickly Man!]- Updated for ULX v3.51
