You can select a single user in the users list and invoke its deletion using the Delete Configuration Workbench toolbar button.
Delete is protected by an ‘Are you sure?’ dialog.
If the deleted user has access only to the current RPI client, deleting removes their user record from RPI permanently.
If the deleted user has access to more than one RPI client, deleting only removes access to the current client, and does not delete the user’s record.
Note that you cannot delete the currently-logged in user.