On my PC I am using Thunderbird mail, but this may be of help to you. When I click on "Address Book", a window opens that has two choices...Personal Address Book and Collected Addresses. So what you're asking about would be in my Collected Addresses. I can go to that list and delete to my heart's content.
On my Mac, the email program actually has a menu item called "Previous Recipients" in addition to the Address Book. So it is on that list that I would make deletions.
Try to find a list similar to what I've described and I think that will solve your problem.