You may have a local animal shelter or group that has a feral cat program. CAll them and tell them you have a large group in your area. Basically, what seems to work is that they keep feeding the cats, but they also catch 'em and neuter/spay everyone they can find, then release them back. The few that aren't truly feral and are just strays can be adopted into homes. It's a long-term program, but it keeps 3 strays from turning into 30,000 in a few years. That's probably your neighborhood's problem; two stray unfixed cats turned into a bunch.

You know, I would think a few strategically placed sand boxes and occasional maintenance would help the smell considerably. Given a choice, cats will go in a nice spot with loose earth or sand where they can cover up. (Like your freshly tilled garden.) You might have to work at it, but at least it wouldn't smell. Won't help the males spraying to mark territory, though.