I just went and deleted 11 SPAM comments from my last post, and it's happening more frequently, so I'm going to trial a Blogger feature to try and stop it. I just turned on the 'Word Verification' system in Blogger comments. If (when!) you go to comment Blogger will show a word on the screen that you have to type, sort of as a password.
I hope this isn't a pain in the arse, and please let me know if it is!