Let's assume that the unwarranted criticism comes in the shape of an angry message or email, because come on -- it's the Internet age, of course it does.
When you're slapped in the face by the smelly trout of criticism, your knee-jerk reaction is to reply right away, possibly by hunting down the jerk that dared affront you and introducing their face to your knee. It is important to resist this temptation.
What, where did you think the term comes from?
At this point, replying is the absolute stupidest move you can possibly make. There are precious few things in life that can't be ruined to a magnificent degree with an ululating phone call or a hasty "Oh, you think I suck on a professional level? That is curiously similar to what I told your mom last night" reply to the unpleasant email from the boss. The important thing is to remember these simple things:
First, step away from the keyboard. Seriously, back the fuck down. Right now, there's nothing you can do with that thing that will not get your ass in trouble. Wait at least an hour before replying, it'll give your brain time to sand the edges off your rage. To take your mind off the topic for an hour or so, maybe pop on over to the nearest sports store and idly pick up some random items. A ski mask, for example. I mean, obviously that's just one off the top of my head, but yeah, pick it up. Shopping can be very therapeutic; it'll soothe you.
Go to a spa. Or, just repeatedly say the word "spa."