Glyphosate and patience. Apply at the required strength and be patient. Remove flowerheads before they set seed. The reasons most people find that glyphosate weedkiller doesn’t work are to do with how the weedkiller works. It has to travel right through the plant. If applied at too high a strength, the leaves will burn and wither protecting the roots which will then reshoot. Equally if you cut down the plants when the tops look dead, this will protect the root and the root will regrow. You might have to leave it the whole season to die but it will work. Digging up just breaks up the root and every tiny piece will regrow. You might get some regrowth next year but it will be less, just do the same again. Glyphosate and patience.