I find the "ignoring" strategy works quite well. I can usually get the gist of the more ridiculous posts from the replies of those who can be bothered to read the drivel their posts - so will respond to them instead (reasonably of course
)
I've found that the technique of making a general statement, not obviously aimed at the stupidity, but highlighting the difference by comparison works well. I have to resist taking the passive aggression too far - but sometimes they do really irritate 
With certain posters, I'll also make a point of referring to them as "he": no idea if they are or aren't but their aggression definitely seems to have elements of testosterone. 