Did Hertz give you the contract when you picked up the car? Usually you have to initial next to the insurance clause to either decline it or to accept it. Are your initial's in the "decline" spot?
If so, if it were me I would make a copy and send a letter to HERTZ certified mail return receipt with a copy of the contract and a copy of the billing statement showing that they charged your card for a declined service and strongly request in writing to have it removed immediately. If it were me I would also contact the cc company to have the charge removed since you have the proof.
If you don't have the proof, it will be tough to fight. You can still send the letter but your chances of getting the charge removed are slim to none.