Answer: Self seeding has been the bane of these plants since they were introduced to the garden! Hand pulliung is easier the sooner it is accomplished so the plants are still small, and is also easiest when the soil is wet.
You could carefully spot treat (perhaps using the wipe on method) with an herbicide containing glyphosate, but do read the label carefully with regard to correct application and safety measures. This is a nonselective herbicide and so must not be allowed to contact desirable plants.
In future, you can prevent the plants from setting seed by deadheading them after they bloom, or by removing the seed pods in late fall or early winter before the seeds are released.
Finally, some of the newer named varieties either set seed that is not viable, or set less seed than the older types so you might consider planting some of those.
Q&A Library Searching Tips