You should use a proxy server ideally.
Easily bypassed. Just ask a friendly school kid about what they get up to during IT classes.
Or set a local policy for the guest account giving them a specific IP address
IP addresses are per computer, not per user.
Setting a local policy to run a script would mean you would have to give the user rights to change the IP address, which would mean they could easily change it back.
As I said.... if you're unable to disconnect the cable, then the only answer is a kiosk setup.... that's at least a bit trickier to bypass than a simple proxy.