you can use poison to get an out of control population under control but poison is kind of awful, it only kills a percentage of the rats and then the rest pass on more resistance. then there is the reality of unintended recipients of the poison, any animal that feeds on those rats, like cats, owls, birds of prey etc. if it worked it might be justifiable, but more rats will always take the place of the dead ones. nothing short of cutting off the supply off food is really going to work. you must seal up the coop at night. hardware cloth is the only thing I know of that does the trick, envelope the whole coop and don't leave more than a 1/2" gap to the outside. within a few short days of running out of food, the rats will begin to disperse.