How to encourage neighbors to spay/neuter/feed their pets?!

I live near an apartment complex which is currently a "kitty city". Neighbors have come and gone leaving cats behind. These cats, as well as current pets, have parented numerous litters. Many of these cats and kittens visit my porch looking for food (of course I can't not give them some!) I have three of my own who came to my yard as ferals kittens almost a year ago. They are all spayed/neutered/vaccinated, but must live outdoors as a family member has severe asthma/allergies. How can I encouarge my neighbors to become responsible pet owners and spay/neuter/vaccinate/feed their pets?! I don't want my babies to be injured by them or get sick, and I wish for all these animals to have happy, carefree, full-of-love lives like mine, but I can't take anymore in!! Any advice is appreciated!!!

I would see if you can get a local rescue group or TNR group involved. (TNR= Trap, neuter, return.)

As one person, you might not be able to have an impact in helping all these CATS to a better life, but a group might.... trying to adjust people's morals - that's a whole other bag o' bones - good luck.

I would get in touch with the Feral Cat Coalition and ask what your options are. I know of some people locally who take cats that have no collars and get them s/n and adopt them out. If the cat looks unowned and not cared for then they pick it up take them to their no kill shelter. Sometimes they would s/n an owned cat and take it back. It is a fine line and somewhat extreme but I can understand the frustration.
I am not sure how you approach people. I have found people are very set in their ways when it comes to pet care. You can show them stuff and lead them places but I have found that people get defensive.
You can try to offer to take the cat for them or try to talk to them. Even if you only get a few people to come around it will have a huge impact.