If you know anyone with unaltered cats, please refer them to barcstl.org - which often offers spays and neuters for free or $10. There are other good resources for cheap or free altering in the area as well.
For the stray cats, you pretty much have 2 options. You can try to take part in a Neuter/Release type program (google for one in the area) where rescues seek to alter all stray cats so the reproduction stops. You would trap them in a Hav-a-Heart kennel (I think they're called) the humane kind.
The other option also involves trapping them and then taking them to Animal Control, where they will be held for a short period and put to sleep if not adopted.
A lady on our block was feeding the stray cats, which made them multiply from three to about a dozen. Then she hired a pest control company to trap them and, I imagine, they were taken to animal control or god only knows where. One could sell cats to animal testing companies and the like, I've read.