So I don't know the company you're contacting but this should help: part of GDPR legislation is that you can contact and live email the company has and make your legally mandated demands that your data be deleted under GDPR. If they ask that you create an account to submit a request then you can claim this is unnecessary and obfuscatory to the purpose of deleting account details.
Make sure you tell them they have 30 days. Make sure you ask for all data related to your email, name, identity, phone numbers, whatever.... to be deleted. Quote the parts of GDPR legislation that are relevant to this (it's actually quite short so worth a read), and send them a link to the national authority you will report them through if they don't provide a satisfactory resolution in 30 days.
I did this for Hermes, an atrocious courier company in the UK who I hadn't used for anything for about 7 years. I got bounced around but eventually got a direct email to their data handlers. They demanded I give them a passport to prove my identity. I told them that the email I'm messaging from is more than enough for them to confirm. No response for 20 days (the timer was ticking from when I first emailed anyone at their company with the request) then they deleted my information right before the deadline and confirmed it with me.
Bastards, but if you're committed to reporting them then that seems to be good enough leverage for many crappy companies.