Not sure other than digging them up in early spring. I let mine grow (along with the dandelions), they look so pretty together.

There is nothing you can buy that will even begin to make a dent in the population of wild Violet except making the soil your turf, and the Violets, grow in less conducive to their growth. Have a soil test done by your states USDA Cooperative Extension Service (if I had some idea where you were I could supply a link) and adjust the soil as needed to discourage them. Violets seem to grow more readily in soils with a pH lower than is optimal for most grasses.

Thanks for the comments. Sounds like I should continue to lime and maybe the violets will disappear over time.