In this land of free speech, there's nothing you can do about what people say about you, especially on forums where the posts are anonymous. In my experience, these folks aren't especially interested in facts.
What you can do is to "live well as the best revenge," although I don't like the word revenge. Let's say, live well (and work well) as the best antidote to negative publicity. If you do, the positive reports you receive from satisfied customers, admirers, etc. will far outweigh the negative ones.
I moderate the comments on my blog so that no spam gets posted. I do, however, post, and address, most "negative" comments, which usually arrive in the form of challenging questions. While the questions may have a tone of 'tude, they deserve to be answered. Being able to handle pot-shots with grace, gratitude and honesty does wonders for my reputation, even while this was not apparently my critics; intention.
Receiving criticism is also an opportunity for self reflection. If it hurts they're right...but only according to us.