There was a guy I heard of who took advantage of the system before "limit x per person/household" became popular.
He'd find local contests, which often by law require you to be able without purchase by mailing a SASE, self-addressed stamped envelope as an entry.
I can't find his name ATM, but I know he won decent amounts of cash, a new car, and more. This would have been... late 80's, early 90's.
Problem is, this guy isn't an easy target to force to pony up. It's not like he's a corporation that announced a prize.
I would CONSIDER contacting Steam about his refusal, since he is connecting himself to their company. It's not good business to call customers cheaters and liars, and renege on a promised prize.
At the very least, he's committing libel by calling you a cheater and DIRECTLY linking to your steam profile, asking others to "yell at you". Steam and most others would consider that harassment, which is against the TOS.