Try putting the email address into the blacklist. Should work like a charm. The only problem of course is that the person can easily change email addresses in which case, you're only playing whack-a-mole.
[. . .] I'm not sure that you need to, but you may want to escape the email address like user\@yahoo.com.
The Blacklist Entry:
hrie\@yahoo.com # The Problem Child
Your comment submission failed for the following reasons:
Your comment could not be submitted due to questionable content: email@example.com
And it's all good.
Different IP addy and, ah, client for his or her entries, usually, but the same ol' email address. Easy enough to circumvent, and no doubt will be if enough MT-Blacklist users make that addition, but in the meantime?
Peace, quiet and no annoying adverts.