[Cicero]

TO the OP: You had no business putting your stuff under your seat. That space is for the passenger behind you to put his feet. It must have been he who complained to the FA otherwise how would the FA have known it was yours. Instead of freeing up overhead space for other people, you should free up the passenger's leg room that you took from him.

TO Butch Cassidy: Whether a ticket is refundable or not, if a passenger feels intimidated by an FA and his supervisor, and he feels like leaving, he just has to get up and walk out of the plane (with his mittens and all) and demand that that the airline takes him on another flight. Just as the airline can throw a threatening passenger off a flight, the passenger can leave if he feels threatened.