That is agood question.... Well if someone obtains the MAC and IP of a guest wireless users, then I would have to say there is no way you can stop that person from getting on.  People have been doing that for a while at hotspots:)  It is a guest network that isn't on your network so the question is... does it matter.  It is the responsibility of the user on the guest network to protect his or her device as I see it.  Anyways... that person would have to wait for the other device to logoff in order to not have any network conflicts.... but then... if they logoff, the unauthorized user would have to logon.