In no event under any plan or otherwise may the obligation between the district and its general obligation bondholders or, in the case of a special tax or assessment district, the obligation between the district and the holders of the county bonds issued on its behalf, be disturbed. If adequate provision is not made for the levy of taxes or for payment of the principal and interest on such bonds, it is the duty of the auditor of the county to levy, and of the treasurer of the county to collect, an ad valorem tax, without limit as to rate or amount, upon all taxable property within the district as it was constituted on the dates those bonds were issued sufficient to pay principal and interest as they become due. Only bondholders or agents or trustees acting on their behalf may proceed at law or in equity to enforce this requirement.