My apologies for resuscitating such old thread; but this is where google took me when trying to rid my Mail.app of that bulk folder. None of the solutions posted in the thread worked for me. I hope the person willing to give out their tooth and other body parts is still all in one piece so he can pay me.
Here's how I did it:
1. right click on a folder under Yahoo! on the left side of the mail.app window.
2. Select the menu item that says "Edit Yahoo..."
3. Select tab 'Advanced'
4.Type "[Yahoo]" in IMAP path prefix
5. Done, close settings window check mailboxes under Yahoo! for successful change.
Charlie7:"eye teeth" please; whatever that is.