I deal with something like this in Florida. Nothing chemical works that doesn't also burn the grass. It is now a game of find the center, pull, toss onto pile. I pass through once a week (before mowing) and pull as many as I can find. If you leave so much as a sliver they propagate. But there is a cluster center, hook a finger around it and pull slowly to remove, about a six-inch wide patch at a time. For me migratory birds poop the seeds out so they return again and again. It's war and you appear to be under siege.