Wild violet has a reputation of being notoriously hard to kill. But most of that reputation is because of the use of the wrong type of herbicide. Wild violet is only responsive to post-emergent broad leaf herbicides that contains triclopyr and is listed as safe to use on lawns. This type of herbicide is much more effective on wild violets than any other. However, wild violet is stubborn and it will likely take more than one application applied over more than one season to get rid of the plant for good.
Mow your lawn and the wild violet. By cutting the wild violet back, you will force it to start growing rapidly. Perennial weeds like wild violet must be actively growing when sprayed or else the herbicide will not be drawn down into the weed's roots.
Spray the wild violet once it has grown 3 to 4 inches. Coat all of the plant tissue, but stop just before the herbicide drips off of the plant. In roughly two weeks, most of the wild violet will have wilted and turned yellow.
Spray the wild violet again three weeks after the first application (or at the frequency dictated by your herbicide's manufacturer). The second application should kill the majority of this season's wild violet. If not, spray again as necessary at the intervals specified by your herbicide's manufacturer.
Hand pull or dig wild violets from green lawns or in conjunction with herbicide applications. When you dig up the weed, be sure to take as much of its root system with it as possible. If you persistently pull wild violet season after season, you can eliminate it from your yard.