It's not so much a rebuttal, but ask if they think stealing has any relation to depriving another person of something. Imo, they have a correct, though extremely narrow, definition of stealing that doesn't leave any nuance for comparing different kinds of stealing. Piracy, or as they would say 'stealing digital media' is not a kind of stealing that deprives another person of that thing, so clearly it's somewhat different than stealing money or physical property.

If they aren't willing to entertain that there are different kinds of stealing then they're ignorant of reality and it might not be worth your time to try to change their mind.