You can delete your Rate my Voice account and everything in it. There are two ways: inside the app, or by email if you no longer have it installed.
Write to support@ratemyvoice.site from any address, telling us the nickname on the account. If the account has a recovery email saved, send it from that address; otherwise we may ask you something only the account holder would know, so that nobody can delete someone else's account.
Everything tied to the account, permanently:
Encrypted database backups roll over on their own schedule, and anonymous usage statistics that cannot be traced back to you are kept as aggregates. Neither can be used to reconstruct your account.
Deletion cannot be undone. There is no restore, and the nickname is released for anyone else to take.