I'm not certain whether this is Leucanthemum (shasta daisy) or Echinacea White Swan, the flowers are a bit far gone to tell, but either way, cut all flowered stems out - it will disappear completely over winter, so you can cut the foliage down to the ground later on, though you don't have to.

It's Leucanthemum. Echinacea discs are more globular and spiny looking. I'll second Bamboo on treatment, too.

Yes I go for Leucanthemum too. treatment as described by bamboo.