This document explains the behavior of the Remove User from the System button on the Users tab in the KB Admin Tools.
The Remove User from the System button allows you to delete a user's profile and access privileges from all KB groups even if the user belongs to more than one group. This option is only available if the current group is the user's default group.
Note that User removal from a group cannot be completed if the User owns anything in the that group.
Likewise, user removal from the KB system cannot be completed if the User owns anything in the system (e.g. in any other KB group). Documents may need to be transferred to new owners before a user can be removed from a group and/or the system.