I've reported the problem to Support and was told "Thank you for the report; I have removed that account from our system. At this time, however, there is no way to delete a "like" from a post on your site. Our apologies for this inconvenience, as we know it's certainly not a good thing to have a spam like attached to one of your posts. If you need anything else - or have any further questions - please let us know."
So basically the spammers seem to have found a way of getting their details onto our blogs, and we have no way of removing them. I've actually turned off the Like button, which at least stops the spam from appearing, but of course to some bloggers this is a very unattractive option.