How to disable guest user (after lion update)

System Preferences > Users & Groups

I suspect thought that the Guest will already be disabled but you want to hide the user from the login/lock screen. If that's the case, go to:

System Preferences > Security & Privacy > General > Advanced > Disable restarting to Safari when screen is locked

However, this is apparently a security feature so if your computer is stolen the thief will restart into Safari using the Guest account, connect to the Internet therefore allowing Find My Mac to trace the computer location.