How to stop BigPond Email sign-in Popups

I had a BigPond email address which I abandoned due to overwhelming scam messages. I have constant pop up requests to log into that old email address but I do not have the password. I have tried the "forgot password" method but my phone and old bigpond email is not recognised as having an account. I would like to stop getting the Sign In popup which interrupts every time I open my laptop.

[Re-Titled by Moderator]