Wild onions and wild garlic are difficult to control in lawns. As you have discovered, digging them up is often impractical, because any small bulblets left behind will resprout. You can spray with a herbicide such as Weed-B-Gon MAX, but that, too usually requires repeat applications several years in a row to eradicate the wild onions. The best time to treat them is as soon as the leaves emerge in the spring, though they can be sprayed any time that they are actively growing. Spot treating with Roundup can work, too, but Roundup will kill your desirable grass.