Can't say I've ever been there, amigo--I don't think anyone's ever accused me of stealing their story, which is a bit surprising, really. But I think if they did I'd just politely point out that ideas are a dime a dozen and parallel development happens all the time, and tell them I'm sorry they think I somehow stole their idea/story/material but that certainly isn't the case. There's nothing more you can do, really, but stay calm and polite and not let them bait you or make you lose your temper. Because, honestly, unless they have some kind of proof and take you to court the worst that can happen is it can damage your reputation--and that's a lot more likely from blowing your top at them than from them running around saying you ripped them off.