I freely admit, I started writing fiction in part to vent. I was one of those teenagers who would have worn nothing but black if my parents had let me, and who composed odes to dying trees, wolves in the distance, and dreamed of the day when I’d tell my peers and tormentors (there was a great deal of overlap in the categories) to go jump in a lake and have the super powers or Mecha-style armor to make it happen.
I still do that, the writing bit, just more subtly now.
While in grad-school I vented with a story that later got circulated to a small, trustworthy group of fellow sufferers. I managed to nail every quirk and irritating habit rolled into one character. No, the character didn’t come to a bad end, but it was fun to see just how annoying the rest of the universe would find those behaviors. Short answer: very.
The way to do this without getting sued, or cornered in a dark alley by the individual you are irritated with, is to focus on the behavior. As Sarah, Amanda, and others have repeated frequently, you should not include real people as real people UNLESS you have very explicit permission and approval from them, preferably in writing. There is no way I’d list [professor] or [former supervisor] in a book by name, although I still have plans for a certain co-worker, later supervisor, unless that individual has gone to prison by now. Even then, no names, no using his/her/its physical description, nothing that would allow someone else to say, “Oh my gosh, that’s [name!]” and be correct.
There are behaviors, however, that drive me up the proverbial tree. I was at an academic event and found myself sequestered away from the students, as were all the other teachers. I had not brought ear-plugs. This proved to be an error, because two of the other teachers bloviated for seven hours straight, not including lunch. Even tactful suggestions that these people quit loudly airing their district’s dirty laundry and families’ gossip didn’t register. That behavior will appear in a short story and will lead to an unfortunate encounter with wildlife.
Federal administrators who impose random roadblocks on researchers? Skewered. Obnoxious supervisors? Dispatched into dimensions unknown by nasty eldrich creatures. But always focus on the behavior, not the individual. We’ve all known someone who held onto information because they thought it gave them power and made them a vital part of the corporate food chain, untouchable and near god-like. Actually, what it made them was a nuisance for the rest of us trying to get work done. We’ve all collided with rude clerks, crude customers, teachers who thought they walked on water, children with parents who just knew they were raising a Perfect Child™ who did no wrong…
Use the behavior, not the person. Someone who knows all, and keeps talking when other people are backing away at ever faster speeds, the guy who refuses to look behind him because he knows more than the local guide does about the local wildlife and customs… That he’s based on the receptionist at your daughter’s dentist’s office is your little secret.