Any enzyme-based washing power (like BioZet) from your supermarket will break down the protein in the urine and stop it from smelling. It works in the same way that the "UrineOff" product works, but works out a lot cheaper for you. The only tricky part will be working out the easiest way to get it on the bag in the first place. Usually we recommend it at our clinic for people who have had cats spray on carpets etc, in which case we recommend making the powder in to a bit of a slurry with water and scrubbing it in a bit, then washing it out. With the bag you might find it easiest to perhaps make up a concentrated solution in a spray bottle and spray it onto the bag...

Good luck!!!