This is my solution to excluding malicious commenters:
In the WordPress Web management UI
click on “options” in the mid-blue top toolbar
click on “discussion” in the dark blue lower toolbar
In the section titled “Before a comment appears” select the checkbox
“Comment author must have a previously approved comment “
This means that people who have a comment already posted see no change when they post a second or 100th comment. If someone does not have a comment already showing on your blog then their comment is ‘held for moderation’.
This is working better than ‘banning’ a specific email, IP, name or combination of these commenter-identifiers because no matter what the person submits, it is held for moderation if that commenter has not posted with those personal details before.
The downside is that you do have to approve all ‘first time’ comments, this is a small price to pay and may stop random people using your blog to publish abusive content.
I don’t understand vkaryl’s suggestion about deleting users because in my admin, I am the only user, so I can’t delete anyone.
Hope that helps, let me know ??