Stop and Assess. You've read a comment and you're feeling angry/offended/upset. Before doing anything, think about why. What buttons of yours might be being pushed? What are your thoughts about the feedback, and are they realistic? Are there alternative interpretations to your initial reaction? Often, when something someone says (or writes) makes us have an extreme reaction, it's hitting a nerve that may not even be directly related to the current situation. We may be misconstruing the feedback or amplifying its negativity because of this. Try to be aware of what might be going on in your head.
Step Away. If you receive negative feedback and its upsetting, step away from the screen and process your reaction (above). Go do something else or talk about it with someone to see what their thoughts are on the matter, and what advice they may have.
Never Reply Angry. I can think of emails that I've written in anger and am very thankful I never sent. If you find that you've written an angry reply, save it to your drafts and step away for a while. Give yourself at least 20 minutes to think about things. The more time the better, if possible. Replying angry is how flame wars get going on message board threads or Facebook walls.
Reach Out. If possible, reach out in private to who is giving the negative feedback to discuss. Be open to constructive criticism. This doesn't mean you need to cave and agree with feedback you don't agree with, but try to engage in a dialogue to maybe find some middle ground.
Beware of Drama Seekers. Some people simply thrive on drama. Engaging with them does little good and will likely just suck you into the drama vortex. Ask yourself if its worth getting upset over or if its better to ignore the person. There's a saying I use "don't pick up the rope." Meaning there's no tug-o-war if you don't have a person on each end of the line pulling. If you sense a drama seeker is at work, don't pick up that rope.