I can suggest the use of a little electronic device that emits an ultrasonic noise, not audible to humans, that will keep rodents out of the area. We use them in our second home in France, which particularly during the summertiime mice and other rodents tended to make theirs, until we put in a couple of these devices. You should be able to pick them up at garden or hardware stores and they are an 'environmentally sound' approach, compared to using something like poison (which BTW also cause a horrific death to the animals concerned).