Server :: Logged Users Without Processes?

I try to write a script which would kill processes of users who are not logged in. My approach is to find out what users are logged in and then kill processes of all nonsystem users who fail the test of being logged. I use `w` for finding all logged in users, but apparently there are users on the list which `w` gives me who own absolutely no process in the output of `ps aux`. How do I log off those users, since killing their processes wont work (since they own no processes)?