This document explains the behavior of the Remove User from This Group button on the Users tab in the KB Admin Tools.
The Remove User from This Group button allows you to delete a user's access privileges from the current group.
If the current group is the User's:
this removes the User from the KB System altogether.
If the current group is the User's default group and the User belongs to other KB groups, this removes the User's access privileges from the current group only and resets the User's default group to the lowest KB group ID (with which the user is associated). The resulting page will indicate the user's new default group name.
The Remove User from This Group button is only enabled when the User does not own any Documents or News items, etc.