turn off an email account we can no longer access on the iPad
Deleting your old email address off the ipad will not delete the account itself it merely removes the username / password from the iPad so it will not attempt to log in. But since you have changed ISPs anyway this is a moot point. Caution however make sure you have a backup of your archived mail in your possession on another PC or backup device before you do the following.

On your iPad go Settings>Accounts &amp; Passwords

Select the mail account you want to remove

Scroll right down to where it says "Delete Account" in Red and click that - and again to confirm

you are done, your iPad will not try and access the account.