Use .5" hardware cloth to skirt the area (under the coop?) attach to coop and fold a foot or so, at the bottom away from coop/area. Then either bury lengths of the hardware cloth/or place on ground surface (stake/weigh down) - should have about 2.5' of hardware cloth on/underground all around area. All windows/openings-cracks in coop should be covered with hardware cloth. All food sources should be removed at night. Fill in all holes.
That said, unless you make your coop/home utterly disgusting to the rats (tough to do) they'll breed like, well, rats and you'll be praying for the Pied Piper.
Very good luck, let us know how the kinder/gentler population and chook protection control program works out for you.