Is it possible to create a genophage/xenophage that only does x to _____ species?
Say for example I hate gray squirrels more than the Abert's squirrel so I want to make all the gray squirrels sterile or to just die of heart attacks before they are old enough to breed. Is it possible? Would it be genetic or nanotechnological? What would be the implications of this? What if I wanted only gray squirrels gone but it kills or sterilizes 30 different species of squirrel?

I want it to target only one thing, the gray squirrel.