There's no way to do it that is fast or easy. You can either hand pull the grass or wait for it to die out (assuming it's a shady spot - and as such, being rather unsuitable for grass, the violets will win the battle eventually). I only say that because Viola blanda grows wild in just one very shady, moist area with grass in my own yard, and the violets are clearly gaining the upper hand with the passage of time. You can certainly mow the grass to keep things neat in the meantime; the violets won't mind a bit. Then you could pull out grass, even if only a little at a time, until it's eventually gone. Good luck, they're worth it!

Stefan

Theoretically, you need to use a graminicide ("grass killer") - such as Poast, Select, Fusion, Fusilade, etc. However, I have no idea which one to recommend in your situation; many have been developed for commercial grass crops such as maize (corn).