Spay Neuter Release (SNR) is a program accepted across the country as a way to deal with feral cat colonies. If you can catch, spay/neuter, and release them, you can reduce the growth of the colony. It's humane, it's all many are able to do to address the problem, and it reduces the rate at which they can multiply and the suffering that results from so many animals without proper care.
Memphis' Animal Protection Association at 324-3202 can provide info on low cost spay/neutering services.