So sorry to hear this!! Unfortunately, SOME Neighbours, just, want ALL their own way!! I know someone who went next door, to, complain about a microwave 'pinging'!! As someone else, rightly, said, IF you 'back down', it, really WON'T be, TOO long, before, they are 'on' at you, over something else!! Such a 'shame', you have 'landed up' with a Neighbour like this!! IF it were me, I would, have, some kind of security camera installed, get, hopefully, a CLEAR 'image', of, them 'dumping' the leaves, back, to you!! You could, then, perhaps, at 'threaten' them, with, some kind of civil?? action, or, calling the Police!! You, also, could, offer to show them the video 'evidence' of the above, BEFORE, you take it any further!!