why do you want to burn it? its a pain for the neighbours and not particularly safe. You can cut down your pampas grass in late winter early spring in colder regions or earlier where it is warmer. All the old stuff does id protect the crown and new shoots over winter
It needs to be cut down every year. I do mine in March. I use hedge cutters and get is as low as possible. You wont get it to the floor because of the huge crown. New growth emerges. If its too big and you find it troublesome, its better to dig it up rather than put up with it. My pampas is the smaller species, but its still big enough! I reckon that if you cut it back hard now, it would still catch up and have plumes later on.