What I would do is create a super limited user account which serves only to force people to signup (all they can do is open a web browser that allows them to access only 1 website).
It probably wouldn't log them off automatically, but they wouldn't be unable to do anything else in that user account.
Many kiosks are setup like this... very restricted user account that can't do anything except browse 1 domain (e.g. the store's website). They have a blank desktop and just a shortcut to internet explorer, and the browser denies them access to menu bars etc. and only works on 1 domain.
I would look at how people setup kiosks with windows and work from there...