If ****ing humans could only be arsed to spay/neuter their domesticate animals, this kind of problem wouldn't arise in the first place.
They need to do a massive trap, spay/neuter/release, and start cracking down on people who don't spay/neuter their own pets, or allow them to roam.
That would take care of the problem in the same amount of time as it will take to kill 2 million cats, only to have the problem resurge time after time.