Firstly, I take great pleasure in saying "GOOD FOR YOU, NYZKI!", for doing something so proactive and using your frustration and anger in a constructive way!! And it was SO good of you to take in the one cat! (if only the ignorant ones would acknowledge those good works...) I agree - it's totally maddening and a thankless 'job'.
If you know the animal protection laws in your location and whether abandonment is considered a crime (it usually is in most places in the US and Canada), and if you know your old neighbour's last name, you
could still try reporting the abandonment, since you actually [i[have[/i] their cat as proof.....not that it's any guarantee they'd even be charged, if they could even be found because often the authorities don't care, either, or don't have the manpower to track people down. However, there's usually a paper trail when people move. So it might be worth a shot.....which is what those people deserve, if you get my drift!

The same garbage happens all the time around where I live. Heck, even our city doesn't care about the
animals - they only want to 'cleanse the streets'....pretty much like in China, but w/o ending up eating them.

"Out of sight, out of mind" seems to be the motto around here. I would hope, though, that your anger doesn't extend to the neglected, unfixed ones....it's not their fault that they do what they have to to survive, and what comes naturally, when they're not fixed.
Some color pictures taken from the 'net from abandonment stories could be used in your flyers to drive home the point. There are TONS of them out there. However, it usually takes the equivalent of a hammer to the head to budge people who just don't
want to act responsibly to start really giving a tinker's damn.
Having said that, I was more than surprised when one of our neighbours
actually got her cat spayed (!!) after I'd kept plugging away at all the reasons WHY she ought to....plus, her cat repeatedly came a long way just to be at OUR place instead and I'd 'bug' them to come and pick her up each time. And later on, I must have actually scared her into keeping her cat (who'd become a super girlfriend of mine!) indoors for the balance of the winter (although what I'd actually suggested this woman do was give her cat outdoor, SUPERVISED time, not just lock her up forevermore, since she'd been allowed to run around at large ever since they'd gotten her & NEVER gets played with at home, poor girl!) She kept thinking I wanted to STEAL her cat.....just because I love animals.....the idiot. This happened when I'd quite innocently suggested she consider giving me a little bit of her cat's food to keep on hand for when her cat got out and came over to our place, honestly, just because I was worried strange food might upset her tummy should I have to feed her again! (we often weren't called for HOURS or even until the next day, to see if we had her in our care) So, she's been indoors ever since, except for once this week....and I'm still just hoping she's still alive, as she never hoofed down to our place for some reason. Even this situation isn't ideal for this cat, but it's better than her ending up injured or dead somewhere, due to neglectful care.
I'd be harping at those other neighbours to not DARE give any of that litter away either w/o PAYMENT (so the animals' lives are seen to be of value) OR w/o having them all fixed first. Tell them some real-life horror stories, so they won't want to be seen as being contributors to such fates and then keep asking them "When are they going to be fixed?" And IF they're averse to paying for the operations themselves, find out if there are any organizations who might subsidize the fees, IF they really are low-income enough to qualify.
So my point is, never give up entirely as you might be surprised when someone actually listens!