Permanently, as in not until new seeds sprout? If so, then use something with glyphosphate in it, carefully avoiding spraying the leaves of anything you want to keep. No wind and a handy piece of cardboard are essentials.

Permanently, as in bare ground for the next ten years? Well there are various soil sterilants you can use, but be VERY cautious--almost all of those products will kill things that you didn't intend to kill, and some will go on to kill your neighbors' landscaping, the next house down the row, and the park 3 blocks downstream!

A little more info as to the situation and intent would help us help you in this.

If guest intends to pave this area and not grow anything in it for several years then sodium chlorate (is it still avaiable) could work. Use a watering can and the large droplets should not drift as much as a fine spray.
I am assuming that 'lots of grass' means a lawn or similar.

Or get a female dog to pee on it, ours has murdered our grass dead in its tracks !!

Hmm...I guess Bluegrass doesn't come back from that as well as my neighbors Bermucagrass does! : D

The best way (environmentally speaking) is to dig it up. Cut it short and then cut the turfs into small squares (as big as you can carry) then pile them upside down in an area where they can be left for 12 months. This will give you one of the best soil improver's you can get.