It will not be easy. There are a couple of approaches:

1. Tediously cut out all the Imperata foliage from within the plants it has invaded. You may have to do this several times to get it to stop coming back. If there are plants with thick growths of Imperata, you can use a strong herbicide, like Roundup, carefully applied by hand to the Imperata foliage only. This is best done by using a cotton glove, over a surgical exam glove. Dip two fingers into the herbicide, and wipe it onto the Imperata foliage, being careful not to touch the other plant's foliage.

2. Isolate sections of the invaded plant, and carefully dig out divisions, avoiding any of the Imperata's root system. Transplant these divisions into pots, and watch for Imperata growth. If you get any, use the same herbicide treatment listed above.

Good luck. When this stuff reverts to green, it's wildly invasive.

Yes, it should only be planted where it can be contained (by a pot, wide barrier, or perhaps a lawnmower). Too bad it doesn't come with that warning.
In my case, I just pulled it out by hand, I saw the way it was growing and got it before it was too terrible...and just kept at it till it stopped coming back. I think I managed to kill it all one season.

Thank you so much for your advices. We moved invaded plants, dug out the blood grass as much as we could, and are now checking the area daily and pull new growths out when we see them. Hopefully, they will stop coming up next season.