I have a T-shirt that reads, "You're dangerously close to getting killed off in my novel". Killing real-life people off in effigy by way of a character you've written is the best therapy for an annoyed writer, as it typically leave no mess, no clean-up, and if done properly results in no lawsuits against the author.
Admit it, writers--you've done this. Heck, I did it when I first started writing fiction in grade school. Was it childish? Yes. I was twelve. Was it amateurish? Yes. I was twelve, and had not yet gotten the hang of things like changing names to protect the innocent. I wrote thinly veiled vengeance fantasies that today would be termed hate-fics, and not one of them has ever seen the light of day. It was my therapy, and it was my way of coping with the difficulties of life.
Doing it this way as a professional writer can be dangerous, though. M. Night Shyamalan is reported to have done something like this with one of his critics, writing a thinly-veiled version of him into his Lady in the Water as The Guy Who Is Wrong About Everything. Michael Crichton, too, wrote a critical reviewer into his novel NeXt, as a politician with the same name and from the same college, only with a small manhood and an accusation of having raped a four-year-old. Both Shyamalan and Crichton came off as childish and petty with these character assassinations, even (especially) if the characters bore only the vaguest passing resemblance to the originals.
The trick to doing this properly is not to just drop your target into your book without even a name change, even (especially) if you write him as The Worst Human Being In The History Of Civilization. If you write speculative fiction, this is a bit easier, but still, step carefully. Get creative with the name changes, not just changing a couple consonants at the beginning of the surname. Make your target realistically obnoxious, so the sympathy is all on your protagonist (you do have a relatively sympathetic protagonist, don't you?) Then, when the pot is well-stirred so that your chosen victim is recognizable only to you, have fun.
That jerk that cut you off in traffic? His car gets stepped on by the monster of the week. The co-worker that nobody likes who's decided you and he are best buds? A stampede of a chase spills his coffee, wrecks his laptop, and covers him in grime. Your ex? Riddled by arrows while trying to attack the hero's army. Your current squeeze's psycho ex who is determined to ruin your happiness together? Eaten by something horrible. A narcissistic acquaintance for whom ruining your entire life is considered an average Tuesday? Eaten by something even more horrible, or taken for study by something from beyond sanity and vivisected alive if you're not feeling merciful.
Doing this is the writer's equivalent of coming home from a rotten day, plugging in a first person shooter, and mowing down wave after wave of virtual enemies. If done well, it can be seamlessly therapeutic, relevant to your novel, and equally satisfying to your reader to see That Jerk get their comeuppance, even if they don't know That Jerk from Adam beyond the story. Done badly, though, it comes off as a tantrum in text, and could get you in serious trouble. So get your therapeutic deaths out of your system, writers, but step carefully. Bad writing is still bad writing.