Can Lion's Mail.app unsubscribe from Gmail IMAP folders?
One of the ways to do this is in the gmail settings. Now that Advanced IMAP controls graduated from labs, there's nothing extra to install. Go into the web interface for Gmail and go into settings. Go into the Labels tab and you should have a listing of all your labels with some options. On the far right side is a tick box that says "Show in IMAP" If this is off then Mail shouldn't be able to get at that folder.