MT-Blacklist does an excellent job of preventing comment spam here. However, one spammer’s posts were getting through: the one common factor is an email address: bob@y####o.com. The problem with blocking these is that the #### is a different 2-4 digit string each time.
This kind of problem shows why regular expressions can be added as blacklist entries. I finally got fed up enough to look up how to write a regular expression to block these comment spams: