As you're not a TalkTalk Consumer customer and don't have an active MyAccount then the self help email deletion via MyAccount isn't available to you.
But you still do have access to the TalkTalk Data Protection Team.
Understanding your data rights (GDPR). Your Right to Erasure is the relevant section for the DP Team to take your instruction to delete the email address and mailbox.
Bear in mind they'll need sufficient proof that you are the registered user of the email address. Proof of who you are and a postal address and 'phone number to link you to the initial registration of the email address. This is just to make sure they're not deleting an email address registered to someone else.
Let us know how you get on. You should receive written confirmation to your home postal address normally within 2 weeks but allowing up to 30 days.
