Removing users from all groups does not remove them from user pickers

According to http://confluence.atlassian.com/display/JIRA/Deleting+a+user+account, removing a user from all groups should in turn remove them from all user pickers. We haven't seen this work in practice though, and end up just prepending their name with "ZZ - " to ensure they get bumped to the end of the user picker lists.