Don't bother pruning the stuff ,if the plant is far enough away from anything combustable ,then set it alight !!!!! this is the ROYAL HORTICULTURAL SOCIETYS recommended way to deal with pampas grass...the next year the plant will have lots of new young shoots and stems..
Usually you don't cut pampas grass back till early spring, late winter. It's something to do just before the grass starts greening up! Cut it back now and you do chance it winterkilling a bit. tho iv'e never seen one totally die. If it's hitting a house, as many do because they get HUGE over the years, it may be time to just remove it completly.
I use the pull type tie down straps that are 6 feet long or so. Hook one end around a bunch of stalks and go around it gathering it up as you go until you can hook the strap to itself. yank it tight and use gas powered hedge trimmers to cut it off about 10" to 12" high. Chainsaw works well too, but dosn't leave the "stump" as pretty, and is pretty hard on the chain as well.