There is a really interesting post over on ProBlogger about people who use the commenting facility on blogs to slander. Darren has aked people to contribute their thoughts on what they think on the subject and it raises various legal points but interestingly some people think that it's silly for people to sue based on what has been put in comments.
I think that it's really dependent on the subject matter because suing is costly and time sapping so no-one should become sue-happy over their comments. But if a comment defames the author of the blog or people who have contributed into the comments box, to the point where what has been written could be perceived as something that could damage reputation or cause serious emotional upset, I say go for it, particularly when the offender continues to do it and doesn't take the hint.
Some people (mostly the offenders) seem to think that blogging is exempt from the normal rules of behavior and it really isn't. Yes it's supposed to be fun and community building, but there are people out there that do not know how to draw the line (blog trolls) and everyone has the right to be protected from people who have nothing better to do with their time but harrass and defame.