I never comment on denialists blogs. Why? There are many reasons.
1. Denialists can often edit comments in an intellectually dishonest way, making you appear in a bad light, giving them the opportunity to play the martyr card.
2. The regular readers of denialists blogs are probably denialists as well, so the probability of convincing them is minuscule. The fence sitters probably do not read comments to begin with.
3. Denialists tend to be very steadfast in their position and they usually do not listen or respond to counterarguments. It is a futile discussion, since very little actual intellectual exchange is taking place.
So, what to do if you come across a post on a denialist blog that you want to take to task? You can post on a forum you enjoy visiting to equip like-minded individuals with your refutation (and they can help you as well with other arguments), or make a refutation on your own blog. This will work as a more lasting impression on the blogosphere and more fully develop your arguments with links and references without getting caught in the spam filters.