Stop users with roaming profiles logging in more than once

Hi,

Have a problem with a couple of users (with roaming profiles/folder redirection) not logging out of one computer before logging into another.

Is there a way to stop them logging in more than once at a time - force them to log out first before re-logging in on another computer?

I have had a look through user settings and group policy but cannot seem to find anything to cover this issue.

I would really appreciate any advice to this problem.

Thanks.

(Single domain Server 2003 network with XP pro workstations.)