I kill everything with MMS. It has been used to kill mold in New Orleans houses that were underwater, in drinking water, even to cleansing blood before transfusions, lots of other things it kills. It's even taken internally, which I do. Also put it on cutting boards when I'm cutting meats and chicken and veggies, cleanses it between different foods.
Another way to sterilize cutting boards and maybe even feet, is to spray them with vinegar, wipe, then spray with peroxide.
But like others have said, white socks, DRY feet..and lots of soap. Unfortunately with new hormones working teen boys get sweaty feet a lot. Wearing flip flops in summer might help air out the feet.