I've had cats for years (although sadly cat-less at the moment
) anyhow, it is a problem that does exist and we can only all do our best to minimise the effects as much as possible.
Collars - cats that don't want to wear a collar will get rid of it quick smart.
Bells - again cats have a way of even getting these off of collars and as has been rightly said on here, they get very good at stalking and not allowing the bell to jingle.
Sonic scarers - supposedly work quite well, I'd give these a go.
Water pistols etc - great idea even indoors and out. Cats hate water and I used a water bottle squirter in my house, to keep them off the kitchen worktops etc. It soon got to the stage where I only had to pick up the bottle and they ran. It was even better when just the sight of the bottle sitting on the kitchen work top saw them slink away.
Keep your cat in at night - this is the best one I have found. Cats love to hunt at night and it is the time when most damage is done to the eco system with regard to wildlife. Keep a litter tray for your cats to use inside (they're your cats, why should anyone else have to put up with their crap in their garden). They will use the litter tray as opposed to ever doing their toilet outside again, which also helps neighbour relations. Curtailing their night time freedom also, eventually, significantly cuts down their hunting habits of a daytime. It takes a while to get the cat into the routine, that night time is for sleeping, not hunting but they adjust quickly and it's a lot fairer on the wildlife than just letting your cat run amok.
I absolutely adore and love cats, have had many many over the years, have lost many to the road too
. However, you can't just say 'oh well it's nature'. Being the owner of any pet comes with a duty to be an informed and responsible pet owner.