Problem solved in about 3 minutes.
Just for future reference, would it be possible to do this from an iPhone? ( lots of thievery going on here )
Or could I just log in on someone elses itunes and deauthorize that way?
I'm worried about someone making a heap of purchases using my stolen computer.
To issue the deathorise all command you need to sign to your account from iTunes running on a computer. You also can't use the feature again for a year, though I gather that sometimes the store support staff can reissue it earlier in exceptional circumstances. I would suggest you don't leave your password saved on any of your devices. That way if a device is stolen they might have access to the current content (though in practice they probably wipe for a quick sale) but they won't be able to make new purchases on your line of credit.