Anyone using this tool in an academic or corporate network (and most other networks) would face disciplinary sanctions. Talking to the network administrator would be the best thing to do.
To immune yourself to it, you would probably need to setup a static arp entry for the router, block all broadcast traffic you send, and send gratuitous ARPs to the router as unicast (frequently). Probably all that after having changed your MAC address. Even then, if the "attacker" managed to poison the forwarding table of the switch you're connected to, your MAC address may leak to it.
You could also counter-attack or use the same tool as him, but you would be liable to the same sanctions as he does.