Controlling the Source of Feral Cats
Responsible pet ownership should be encouraged and fostered so that it happens voluntarily. Irresponsible pet ownership should be frowned up by the community and arguably punished in the courts. An irresponsible attitude towards keeping a domestic cat results in (a) health and welfare issues regarding the cat and (b) quality of life issues (and rarely health issues) regarding the community.
Controlling the source of feral cats comes down to responsible pet ownership. I prefer the phrases, "responsible cat caretaking" or "responsible cat guardianship" as they foster a better attitude towards the cat companion and therefore a more responsible approach towards their welfare.
Responsible cat caretaking is reflected in a cat's needs as summarised in a piece of excellent legislation from Great Britain: Para 2 of section 9 of the Animal Welfare Act 2006. To this section of legislation I think we can add that cat caretakers today (2012) should be required to microchip their cats. Or if not microchip, there should be an alternative (perhaps safer from a health standpoint) to identify the cat as belonging to a certain individual. This would facilitate the return of stray cats to their "owners", if the owners accepted them!
As at 2002, in the United States, only 2% to 3% of all shelter cats are returned to their owners. Microchipping has been recorded as greatly improving this low figure. (See also Microchipping Pets including Cats and Important Micro-chipping Information).
Also responsible cat caretaking should include the requirement that cat caretaking is for the life of the cat. Only in the most exceptional cases should this requirement be withheld. Such a requirement would prevent casual adoptions leading to early relinquishment - see also understanding cat behavior.
The domestic cat that is allowed to go outside should always be neutered or spayed. Or allowed outside into a cat enclosure. I realise that this is asking a lot but whether it happens or not depends on the commitment of the government and local authorities. It could happen. It just means upping the level of priority given to the "feral cat problem". Widespread, subsidised neutering should be available, ideally, for people who need financial assistance.
The last important piece in the jigsaw of creating a high standard of cat caretaking across a large area (nationally or statewide in the USA or other countries) is to initiate some sort of education program for cat caretakers. There is a lot of ignorance about keeping domestic cats. Incorrect expectations based on a lack of knowledge about domestic cat behavior will result in a person abandoning their cat. This may happen soon or after years of mismanagement. People who struggle with caring for cats should have a means to seek advice and training to ameliorate the situation and a service of some sort should be in place to facilitate rehoming in bypassing the cat shelters that so commonly euthanise cats.
The difficulty in instigating change is that it probably needs legislation - the creation of new law. People don't like that and it needs to be enforced. That costs money and money is in short supply (at 2012) and the feral cat is a low priority problem. See People Should be Fined for Abandoning a Cat.