An easy way to prevent this sort of thing is to use two machines. Run Tor and privoxy on one of those machines and allow it to build its circuits and such. The second machine should be configured to use the privoxy instance as an HTTP proxy and should not have a default gateway configured.

(Also, on the first machine, you could use iptables to only permit outgoing traffic from the uid that Tor is running as and to drop everything else, just as an extra precaution.)