You sure? I read the comments on your game and have to assume you hade some kind of sale with claiming enabled and had previously a pay what you want project.
Itch is project based. Someone does not claim files, but projects. A claimed project gives acces to all files, even if the project is marked as paid at a later date. (The exeption are "individually" priced files that no one should ever use, because that mechanic is ill understood by everyone).
Also, if you have ever had a "donation", while the project was pay what you want, those people also can now download any paid files.
A download key is a special URL that gives someone access to a project’s files.
There are a few ways a download key can come into existence:
Someone pays for your project (or purchases a bundle that includes your project)
You, the project owner, generate one from the project edit page
Someone “claims” your project in a 100% off sale/bundle that has claiming enabled