I generally downvote a bad comment that has gotten undue upvotes to reduce its ranking. That is the actual purpose of the downvotes, it's not some childish, vindictive mechanism.

Otherwise (or additionally) if a comment rubs me up the wrong way I may reply in admonishment.

(By the way, this is the first time I've been able to solve the re-captcha in a long time... only the 5th try. Rethink that, especially the initial login that doesn't even have the captcha on it, yet it fails and makes me type in my login again. You know that not everyone has bionic vision and an old 800x600 display to actually see the details in those little images of street signs and stuff? Some of them are pretty obscure, with like one little corner of the object in it etc. At high res, those images are very small)