Two solutions (one of which is terribly unacceptable).
Per the admin Leafo, they can remove the games you purchased from your account. Now, arguably you still have the ability to get the game (you still own it), however you have to re-add it in order to access/download it. ie. Go back to getting the spam. Not really a solution.
Alternately, setup a rule via your email provider or email client to automatically delete any email from postmaster@itch.io (or send it to spam, or other similar location). Not sure what other emails you might miss from itch, but at this point... Do you care?
I went with the latter. I want to be able to get the products I paid for.
I am in the US. Getting slapped into emails like this isn't a cultural thing. It's a scummy business thing. And not to be rude, but the GDPR bit you guys have is new. It's not like companies have been unable to do this in the EU since the birth of the internet. They've just recently had to stop.
The admin here is trying to blame people using a script for the issue; that is intentionally attempting deflection. He understands the issue, but he/she/the company does not want to accept blame for it. A script didn't make these emails happen. Adding purchased items to ones library does so. Doesn't matter if a person used an automated method to acheive this, or manually added the items. The result is the same.
Good luck with your mail to their support. I wouldn't hold my breath, however. And please do report them, if it is actually a GDPR violation.