Thanks for writing. There's not a good, efficient way to stop them from growing. That said, there is a class of herbicides called pre-emergence herbicides; the way these work is that they stop seeds from germinating. Unfortunately, most have a window of how long they'll work --- so you'd need to apply the product on a regular schedule throughout the summer to keep most of the seeds from sprouting.
If your plant is not too large, then you can pick or cut off the faded flowers. That way you'll stop them from producing seeds in the first place.
Other than that, though, there's no good solution, I'm afraid.
---Justin, Senior Garden Editor, BHG.com