I'd just reply to the data controller that you will be needing the following information to file a complaint with your local data authorities
Official company name Restoration adresse Registration number
Prefered means of communication (phone/letter/mail)
This usually does the trick for me. Also mention that you have repeatedly requested your data to be deleted and will gladly verify your account using email but are not willing to give them even more sensitive information than the ones you're asking them to delete.