Sorry..I did miss a bit in the code I posted. Here's an updated version.
menu nicklist {
  $iif($1,Check $1) : $iif(nick isin $ulist($$1),.ruser nick $$1,echo -a None is added)

Right click on a nick in the nicklist, then select Check <nick>
If nick is in the list that matches the nick selected, then the level nick is removed, otherwise you get the echo.

If you right click in the nicklist and there are no nicks selected, then the Check option will not show..additionally it won't show for any menus other than nicklist.