I had so many of them, that I still have seeds from two years ago. The new gardeners I hired came in and chopped down the hundreds of seedlings that had grown to about 6 or 8 inches. You could have one cosmos plant and when the blooms you leave on the plant go to seed, you could pluck the dead-heads off and dry and store the seeds for next year. Also you can just scatter the seeds where-ever else you want them in the garden right then and there. The seedlings are recognizable and easy to remove if you don't want the plant in a particular area.
Cosmos make great cut flowers for the house too.
Sometimes that's the only way. Our neighbors on one side had a beautiful contorted filbert that was dying because they completely neglected it. I was SO frustrated because not enough of it was on our side of the line for me to step in and try to rescue it. The snips I use require a much easier motion - I can snip for a long time without any discomfort at all. You can see it here .