If it's mixed in with other plants, you can't spray it without hitting the other plants.

First put on a disposable exam glove, and then pull an absorbent cotton glove over it. Dip your thumb and index finger into a small cup of concentrated plant killer like Roundup. Apply it only to the foliage of the Leymus. It'll kill the Leymus without harming the other plants. It's tedious, but it works.

I accidentally killed mine by spraying weed grasses near it with Ortho Grass-B-Gon, and the little bit of mist/overspray drifting on a windy day did it. I personally love this plant, and should get some more. But, if you want it dead, this product will do it. You CAN overspray most non-grass plants with the product without damaging them -- the label gives specifics about the plants that can't tolerate it.

Here is a link that might be useful: This will do the job...