Las Vegas where we live is especially plagued with feral cats because of the transient nature of some of our population and because of our warmer winters. In most cases if you try to just eliminate a cat population by destroying the present cats eventually more will come as long as there is some food source whether it be garbage cans or someone feeding outside. The best and most humane way to deal with a population of feral cats is to trap, spay or neuter, vaccinate and release back. Then they should be fed and watered on a regular basis and monitored.
Trapping can be relatively easy with the right equipment.
Humane traps can sometimes be rented or borrowed from animal societies. The traps are a metal grid cage with a trap door at one end and normally another door at the other end in which you can insert a plate of food. The best time to trap is at night which is when most feral cats come out to search for food. If you are feeding a feral cat population, it is best to withhold the food for a day or two before you begin trapping so that the cats are good and hungry. Before you begin trapping you should check with the veterinarians in your area to make sure that they would be willing to neuter feral cats.