Can I delete old unused turbotax accounts associated with my email

Sorry--no.   TurboTax accounts cannot be deleted.   But you can just abandon the accounts you do not use.   If you have not saved copies of the returns you prepared using the old accounts, you should do that---and keep a record for yourself somewhere of how to access those old accounts "just in case."   Otherwise, going forward, stick to one account that you will use consistently.

I believe you can contact customer support to have your account "locked", that will prevent anyone (including you) from opening the account without contacting support again to go through a lengthy identity verification process.  Otherwise, there is no procedure to delete accounts just to clean up.  Set the password to something complicated that is different from all your other passwords, and forget about it.