I own two lovely cats, but they're neutered, and it pisses me off BIG TIME when I read stories like yours. I had a friend who thought it was "cute" that her kitty kept having babies.
There are ways to keep cats in your yard. It is not animal cruelty either to keep them indoors at all times. I don't think there should be hunting seasons for domestic animals... too much potential for abuse... but you should have the right to (humanely) kill anything that crawls on your property.
Both my cats have herpes from ONE encounter with a former roommate's outdoors-loving cat.
God bless you for being a responsible pet owner! What made this whole situation worse is that these cats were no longer domesticated....they were wild and full of fleas and sickly... pathetic. Rounding them up to take to the shelter was a scary thing..I was afraid of getting bit or scratched and getting some sort of creeping crud from them. The last litter I took to the shelter... they didn't think would survive..they were that bad. the people at the shelter told me that where I come from..a town of about 50 houses, had the most stray cats brought in of all the county. We only have a few of them left now..they love my porch (sigh)...but I think they are all males... as last summer their were no new kittens. thank God for that.