Write a letter stating that you filed a flight plan. Then tell them they
waited to long and the tapes had already been erased. Then tell them you
will consider the case closed and any further response will be done through
your lawyer. Tell them if it goes to court you will go after all fee's.
There is no way they can prove you didnt file.
If they respond just keep sending letters with the same thing, then send
them a bill. I bet they eventually get tired of dealing with it and just
drop it.
Have fun with it, and dont fly to Canada.