My block has 8 flats in it with a big communal garden. Each block of 4 has its own front and back door. Bins are kept in the back garden.
Somehow, it's fallen on residents in my side (ie, my block of 4) to sort the bins each week. We're fairly easy about which one of us does it and basically, whoever remembers first does the deed.
But between 3 of us on this side (the other is elderly and we ask her not to as she's hurt herself doing it previously) we're doing the bins for all 8 flats.
We've tried talking to them (they apologise, say they will and never do), and we've now tried separating the bins out so that each side has its own set. They were separated out about 2 weeks ago.
In the small hours of this morning, I was outside having a cig and a resident from the other side lugged his bin bags right across the long garden (their bins are right next to their door, ours next to ours) and put them in ours. We had a quick chat and he mentioned theirs was full.
Today was bin day and nobody from their side has taken their bin to the front to be emptied. One of the other residents from their side has just half-filled our freshly emptied bins 
I don't really know what else to do. We've tried being nice about it and we've tried making it more practical for them by moving the bins closer, we've also tried simply not doing it ourselves, but rubbish just starts getting dumped next to the bins until we relent.
WIBU to send them all postcards stating " There's no such thing as the fucking bin fairy "?
I suspect that would be very unreasonable but I definitely needed a rant about it