I think the worst part of feral cats is how they decimate the natural wildlife. Bird populations can be totally eliminated in their areas especially ground nesting birds like killdeer, quail, pheasant, prairie chicken, turkey, etc. And tree nesting birds aren't safe either as the nestlings...
First thing I would try is the Humane Society, they pick up lots of farm animals that are neglected. If that doesn't work there is a .22 ammunition called .22CB, I call them cat bullets but that is politically incorrect. They are a very quiet, reduced power, short range round good for short...