There is a subscriber to my newsletter whose email address appears to be invalid. Based on the address, it's probably a spammer. I keep getting email notifications from the subscriber's domain that the address has been rejected:
550 5.1.1 <email address removed>: Recipient address rejected: aol.com
I've tried to remove the subscriber both using the Admin panel UI (Marketing->Newsletter Subscribers->Remove), and by removing the entry from the SQL table via phpmyadmin. With both methods, the subscriber is initally removed and the returns with a new subscriber number within an hour or so.
Is there a Magento process that is re-adding the recipient? How do I permanently remove or block this subscriber?