logoff method not working

I am using RS form authentication(from RS sample), after calling Logoff
method, the other RS methods are still working! it seems Logoff method will
not invalid the client cookie! I've tried this on SSL site, not working
either! thanks guys!I know I can just set response.cookie["cookiename"].value = "" to make RS
call failing, just wondering if anyone use logoff method before and how it
works! thanks!