If it is really feral cats they have to go. Sorry I know we all love kitties, but if these are not owned by someone they need to be controlled. They are killing machines - how many critters like pretty birds do you think need to die to keep it alive? Sorry, no compunction and I would not apologize for shooting them. It is common practice in many rural areas and it needs to be.
If it were a native animal I'd say get a live trap and relocate them. Obviously if you can find somewhere to take a cat if you were to trap it where they would feed it go for that.
A pellet gun is a bit light unless it is .22 and pretty fast. Even then I'd prefer you use a real .22 or better to be confident of a humane result.
The antifreeze idea sucks though.