Evening Richard,
I checked the Yahoo mail to find the answer to this.
Your base name cannot be removed as it is used to create disposable email addresses.
I suggest you send them a personal email to try to find a workaround. I couldn't find any real answers using their Mail Help. The above was the only information given.
The way it was done a few years ago had to do with mail box capacity. When it became full, they just didn't pass any more current mail through to the account.
Your only hope is that you reach a limit and the box will be too full to receive any more mail. I think that mail received after that causes the mail to revert back to the sender. This happened to my husband, who went over his free limit two years ago. Senders who had a second email address for him, then tried to contact him via the secondary email address he supplied to them when he registered for their subscriptions; and he was able to give them the new email address, if he wished.
I hope that you can get it resolved.
Blessings and Prayers,
MrsLop