Alright, thanks guys.

I'm going to run out this afternoon and see if I can find one of the
solutions mentioned above. In the meantime, I've boarded up any
crevice or hole that would allow a cat to crawl under. Still have to
figure out what to do about the cats. Yesturday, I was chasing one
through backyards with a shovel. I'm much faster than I thought
because I keapt up pretty well, which was amazing considering I was
wearing big boots and the snow was shin deep. But when all was said
and done it wasn't an effective way to deal with the problem, and an
even worse way to meet the neighbors. As for shooting, I'm not sure
about the local police department's position on discharing firearms in
town. On the plus side, I haven't seen the cats prowling around yet
today so maybe the shovel did the trick after all. Probably not.