Certified GKH Gardening Expert
This invasive species is quite hard to control. Outside of killing everything in the area, it will be impossible. Keeping them cut, completely, as each grows will keep the population to a minimum, but this will not guarantee that it will not return.
Here are some articles that will help:
If all else fails- They are a sustainable food source, since all parts of the plant are edible. Just be sure that what you have is, in fact, a daylily. Unfortunately, your photo did not come through, so I am unable to identify the plant in question.
If you are certain of the id, then this article will offer another way to utilize these, pesky, plants: