I've seen this question posed, but in all of my searching here, it seems no actual solution to this exists (yet.)
Basically, I have one or two people that always leave comment spam, and use the same email address.
Usually, this is where the previous answers turn into threads that really don't deal with this specific issue... so I can help preempt any redundant suggestions from the other threads, I'll post these points.
1) The spammer is pretty tech-unsavvy. I really think email blocking would stop them, or allow me to add an offending email address often enough to a blacklist to get them to stop. (I know people can use different email addresses, and I know its not a bulletproof option, but it WILL work here.)
2) I know that we can force moderation for everything, but I don't want to do this, as 99.5% of comments are legit and askimet is working fine to catch the viagra-like comments.
3) I know I can globally auto-close, turn off, make private or check for previous comments - which isn't what I want to do.
4) The provided spam or moderate blacklist does not block comments by the author's email. It only works on the content in the actual comment text. In my case, the offending comments don't put their email into the content, so it doesn't work. Most of you know this, but you'd be amazed how many answers I've seen that suggest this for blocking based on an author's email.
That's it - looking for a plug-in that will simple ignore/trash/spam a comment based on the author's email addy.
For those that are searching this topic also - so far the closest thing I've seen to handling this in a plug-in called 'Comment Probation'.
Thanks for anything anyone's got on this...