Damn. I'll have to keep this in mind. I can only recommend - updating the game to feature something 'cool' (not an entire new system or overhaul, just something that the old pirated copies won't have) - then delete all the keys? And if necessary, pursue legal action?
In this case the user didn't donate to the game on itch. They downloaded the prerelease version somewhere else, then downloaded the free itch version to gain access to comment, then uploaded the former to mediafire, then posted that link via itch comments in an act of Hate. I see no tools to limit the actions of commenters in any way (links, imagery, etc), so I've resorted to turning them OFF. Going to look into the forum features in the meantime..