Answer:
By far the best solution to population control in feral cats is a Spay and Release program.
Feral cats are well equipped to take care of themselves, especially when they bond together into "colonies". Humans can make their lives easier by feeding them regularly, or capturing them to take to the vet when they need medical care. Feral cat agencies are very useful in coordinating this kind of effort, and providing resources for free vet care.
Of course, the easier their lives, the more they'll reproduce - if they're not spayed. So any responsible feral cat agency has a spay/neuter program. In Berkeley, cats have a triangle-shaped notch in their ear if they've been spayed (it is done while they are under for the surgery and does not hurt them.) So that way, people know which cats still need to be captured and brought in for the procedure.