Disable Guest Account In Ubuntu 11.10
Unlike Ubuntu 11.04, which came with one guest account that you could only access from an active session by default, and which you can disable by un-installing a single package, Ubuntu 11.10 comes with two Guest Sessions enabled; one from the login screen and the other from an active user session.
But the Guest Sessions appear to be so integrated with other components in such a way that I could not yet figure out how to disable them without messing up some other useful features.
So for now, just assume that you cannot disable the Guest Sessions in Ubuntu 11.10. Read more about it and how to create new user accounts here.