However when I go to add a new sender address I get this message " Duplicated email address found"

I have found that this person had previously set up a Zoho Campaign account to see how it worked, but never used it. They tell me that the account is closed.

However since I am getting this error, the account must still be registered on the system somewhere.

How can I ensure that this account is permanently deleted so that I can add them as a new sender address?