Here are three ways: Sirensongs has banished them from her blog (and a short follow-up on online etiquette), Compulsive Confessor asks them to go take a hike if they don't like her blog (I like her first three rules: "1) You not likey blog, you not visit. 2) You not likey blog, you definitely not refresh comment section to add new two cents. 3) You have opinion, very nice, you start own blog."), Blue tries hard to reason with a semi-anon (with what looks like an online version of multiple personalities, all of them humorless) who has been bugging her about dissing "my culture"; this prompts Sharath Rao to give the semi-anon a short lesson in culture as competition, not legislation."
Many bloggers have an explicit comment policy. Developing and maintaining an online community built around one's blog is a lot of work, some parts of it involving unpleasant decisions. Among the Desi blogs, Sepia Mutiny has the most fun comments section; I'm sure the SM folks too have had trouble from anonymous trolls, but they seem to have a good handle on this problem.
In any event, Cory Doctorow's piece on How To Keep Hostile Jerks From Taking Over Your Online Community has some valuable advice. One of the most effective weapons in the bloggers' arsenal is disemvoweling:
... For example, Teresa invented a technique called disemvowelling -- removing the vowels from some or all of a fiery message-board post. The advantage of this is that it leaves the words intact, but requires that you read them very slowly -- so slowly that it takes the sting out of them. And, as Teresa recently explained to me, disemvowelling part of a post lets the rest of the community know what kind of sentiment is and is not socially acceptable.
It's a pity that such a tool is not available on Blogger.com (at least, for its free blogs). It would give the blog owner a wonderful delight that can come only from skewering a troll's comment totally out of shape!
Oh, well ...
Update: BTW, if you want more on how to keep the trolls in check, you can go straight to the expert, whom Doctorow calls the troll whisperer!