Since it is common practice to write and thank someone for the interview (at least if you really want the job,) this is what I would do...
I wanted to take a moment to thank you for the opportunity to interview at your facility. I have however decided to accept an offer which will best suit my vast experience, and allow me generous salary, vacation, and benefits.[even if not true, tee hee] I was actually considering not taking this offer, but after seeing your staff in action, I realized that I made the right decision. I have entirely too much integrity to work with a group of such immature and egotistical people. Now that I have been at my new position, I am even more grateful for the rude treatment I received while in your office, because I am blissfully happy with my new position.
I know you should let it go, burning bridges and all that, but their behavior was so inexcusable. You may even want to embellish and make your new salary 1.5 to 2 x what they offered you. I'd make them think they really missed out on someone (you) good, and something (your new job) good. Maybe that is why they are so mean and miserable. Rub it in with a big smile in sickeningly sweet language on a beautiful card, heck, maybe even send them a big box of chocolates. At least they didn't BS you. If they had done this behind you back, you may not have taken the good job, so you should, in a way
be greateful to them!