[piva00]

If you have documentation about your process (which the email sent to their DPO should be enough) get in contact with the Data Protection Agency of your country for them to start procedures on your behalf.

I've got some help from Sweden's IMY for a similar case, asked for the removal of my data by emailing a GDPR template, got no answer and no resolution by the company in a few months and contacted IMY who solved it (and fined them).

[williamvds]

I unfortunately can't find the emails anywhere, I'll probably try again and keep proper documentation this time.