Mo. Rev. Stat. § 393.280: Defense in case of excessive charges for gas, water, electricity or sewer service.

If it be alleged and established in an action brought in any court for the collection of any charge for gas, electricity, water or sewer service that a price has been demanded in excess of that fixed by the commission or by statute, in the municipality wherein the action arose, no recovery shall be had therein, but the fact that such excessive charges have been made shall be a complete defense to such action.