I can't understand why they're having a collection for someone who has upset so many people and has clearly "left under a cloud". You don't have to "stamp your feet and say no" you just "say no" in a calm rational manner. We have collections all the time at work for people I've never met who are finally leaving after being on long term sick leave, I never contribute as I don't know them. I don't make a big deal of it, I just say "sorry, no, I'm not contributing".
Do you ever have, say, a month at work where nothing happens except you go in, do your job, go home and everyone's happy? Only it seems it's always something - I wouldn't work somewhere where there was always one issue or another!
Assuming that the story is the same from all sides, I'm amazed they have the guts to ask you for money! I would laugh and say "you must be joking". And leave it at that. No need for a scene, but hell would freeze over before I bought a present fro a colleague who threw a drawer at me!