Perhaps you have heard of feral cat clinics, usually in connection with a brief blurb in the local paper or evening news about crazy cat ladies. The fact is, crazy cat ladies typically avoid feral clinics like the plague, because it would mean their supply of new wild kittens to hoard would be jeopardized.
Feral clinics represent a new, some would like to say "more enlightened" school of thought on animal control. In the olden days, and in a depressingly large number of places still, animal control is a euphemism for periodically rounding up strays and putting them down. If that was an effective method, you'd think the dogcatcher, so to speak, would have put himself out of business a long time ago.
The fact is, he hasn't. You can't get every breeding pair in every square mile of an inhabited area short of some kind of weird, paramilitary inch by inch sweep... And even then you'll probably miss some. This is compounded by the fact that aside from the wild population, pets get loose or are turned loose, and given the short reproductive cycle of cats in particular, well, that brings us to point number two.
Cats are territorial, and will spread into new areas in the absence of other cats to defend the territory. So all those cats you just rounded up and put down or adopted out? The cats you missed are going to have babies, and those babies are going to move into all this newly free turf, and they're going to continue to have babies because now they have plenty of resources to grow with. After a few cycles of this, it's easy to end up with a few city blocks populated by miserable, inbred, and particularly sickly cats who are extremely susceptible to common diseases due to poor genetic inheritance, but continue to breed because cats don't know it's not okay to have sex with your siblings.
Now you have a population of feral cats just as or larger than the one you displaced, which carries more disease than it used to, and is therefore a larger risk to people and pets than it was before.
The problem of feral cats is basically a huge, meowling, furred machine involving many, many moving parts, like crazy cat ladies, traffic density, dumpster availability, harshness of winter, efficiency of dogcatchers, and everything else that has an effect on an urban or suburban cat population. The feral cat clinic, rather than attempting to eliminate the mechanism, a losing battle, short-circuits it.
The feral cat clinic model has been proven to work statistically and empirically, and is really intuitively obvious once you stop to think about it. We scrape up the cats from an area, give them vaccinations for the most common fatal diseases, and spay or neuter them (as appropriate of course). Once they've recovered, you release them back to where you caught them.
In the simplest of terms: Healthier cats are better able to defend their turf; fixed cats are unable to breed. Therefore you prevent the spread of additional cats into "inoculated areas" by making it harder for them to win territory, control the current population by having them fixed, and minimize the spread of disease and danger.
As the clinic spreads the inoculated areas across a wider and wider area, they become self-reinforcing. By starting in the most human-dense areas, you garner immediate effect, and by continuing to spread outwards, like an oil slick, you create an eventually un-traversable ring of inoculated areas to protect huge areas with a minimal ongoing expense.
Feral clinics, in places where they operate, are typically staffed by volunteer vets and vet techs, raise funds to self-operate the same way that any charity would, and sometimes receive funding or subsidies from local governments, other pet charities and funds, vaccine manufacturers, and anybody who they can wheedle cash or supplies from. They typically rely on average, concerned citizens to bring feral cats in for treatment by use of humane traps, which are generally provided by the clinic itself free for the asking - you go pick up some empty traps and bring them back with feral cats inside, and they take care of the rest, aside from letting you know when to come get the cats back for re-release.
Even if you're not an animal lover in particular, you should support your local feral clinic, or rally for one if there isn't one. It's not just a matter of preventing needless suffering of wild animals, it's a matter of controlling disease and nuisances that range from ripped garbage bags to debilitating and even deadly diseases for you and your neighbors.
Maintaining an area once inoculated is as simple as keeping a collective eye on the herd and rounding up any new adults for treatment when you start seeing more than a very occasional litter of kittens - in my experience this is on the low end one new adult per year in a fairly large, fully-inoculated area, and as many as half a dozen in an area with high mortality due to natural causes, like ospreys or traffic.
For more information, a good place to start is http://www.feralcat.com/. In some areas, feral clinics go by the name "TNR clinic", for Trap-Neuter-Return, and most have websites easily found by a quick search for "TNR (your town here)"!