If you are not using the email account it is doubtful that your account is sending dubious emails from it.
What is more likely is that your email address has been "spoofed" so that it appears that the dubious emails are coming from that account when in fact they are coming from a completely different email address.
If you get your friend to hover their mouse over the email address of the email or to check the "headers" of the email it will show true email address of where the email originated from.
If this is what is happening it is pointless trying to do anything to your old email account because that is not where the email is coming from therefore deleting the account would not prevent the emails.
In any event if you have not used the "old" BT email address with in 150 days it will automatically be deleted by BT.
See link about BT deleting email accounts and what "spoofing" is.
Why has my email address been suspended or deleted? | BT Help
How do I close or delete an email account? | BT Help
Email spoofing - Wikipedia
All you can do is tell your friend that you no longer use that email account and to treat anything that comes from it as spam and not to open it and just delete it.
