This might be worth a try -
I'm presuming you are seeing your old email address in a drop-down list, or list of options of email address? If so, hover your mouse over the unwanted (old) address and, while it's got blue band over it, press Delete on your keyboard.
I've done this myself in the past to get rid of old options but I may have misunderstood what you are describing.
Please post back to update the outcome :)