You can contact whichever credit reporting bureau has this AU on your report, and ask that they delete the information. They will delete it from your report because typically an AU has no responsibility for the bill. Just let them know, you are no longer an AU and to remove it. That will be no hassle at all ;-)
Oh and the original lendor/creditor often times have no clue because the only thing they do is report it to the credit bureau. The credit bureau's responsibility is to remove erroneous information. And in this case, this would be in error because you were only the AU, and this account was not a joint account.
That's one of the beauty's of AU versus Joint ^wink^.
