Q.How do I get rid of Pampas grass?
I have had pampas grass take over my entire borders and swamp all other plants. I managed to dig them all out a couple of years ago, after burning them as advised. They have come back with a vengeance in the form of thousands of tiny plants, which I have tried to kill off with weedkiller to no avail. A few larger plants have rooted in between patio slabs. I have Googled it and there doesn’t seem to be anyone else with this problem. I can’t plant anything until I know I have eradicated them. Please can you give me some advice.

Here is a great article with some help for you.
https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/pampas-grass-removal.htm