hydrogen peroxide.

Either a cream or paste, that you slather over over the surface. Or you can find big jugs of it at any grocery or drug store for like a dollar.

Put them in a little container like a tupper ware or bowl, slathered in paste or submerged completely in liquid; and then leave it outside in the sunlight for 2 - 6 hours.

A UV growlight will also work.

Once done, rise them off and try scrubbing them again.