Dogs are nice, but elderly folks have trouble taking them out for walks to do their business, especially in winter. If they have a back yard and don’t care about dog crap everywhere, that is different. A nice great Dane, wolf hound, mastiff would be a nice deterrent thoug.
Even small yappy dogs are a noise deterrent and they can be pad trained if taking them out is an issue.
My dachshund is trained to a 3 ft square piece of artificial turf strategically placed in a flower bed at the bottom of the porch. She hates dragging through wet grass and snow. Took her all of 30 seconds to figure out this was her spot.