It's not uncommon for EX to limit on line disputes for AU accounts. They may have a hard time verifying your identity so they want more information. Or, more commonly, being an AU would limit what they might allow you to dispute, and due to privacy concerns, such as account balances, reported payment information, or incorrect information about the account itself.
You should first try to contact the lender and tell them you want to be removed as an AU. Generally they'll comply, and most lenders will also have it removed from your reports on their own. If they don't remove it you can dispute it with a CRA by simply stating you're not financially responsible for the account and never gave permission to become an AU, and/or you don't want to be associated with the account.
