How do I revoke a multisig transaction? Let's say that I am using 2 of 3 multisig. I know how to use signrawtransaction and sendrawtransaction to send the funds if at least 2 of the parties have agreed on the transaction, but I don't know how to revoke it and send the funds back to the first person. 

How do I do that?
You simply can't 'revoke' transactions. If the output defines a 2-of-3 restriction, 2 of the 3 keys must sign the transaction spending it. If no two parties will agree to sign a transaction spending the output, the money is simply lost.