I've just realised that using MT-Blacklist one can kill email addresses that post too. There's this one spammer... 'bob' who seems to post loads, stupidly he has an email address like bob@y8600o.com, where the numbers in the middle are random... I've just added
bob\@y[0-9]*o.com
To my blacklist, in hope that it'll kill bob for good! I hope this might help anyone else who has bob posting to their blogs lots!