I think this is difficult for all and the neighbours sound very unsympathetic but I wonder if this is just a problem during lockdown.
We had a similar issue a few years back, every weekend, every holiday for hours with our neighbours child who had autism/ learning disability. I'm not sure which as the diagnosis changed over the years, neighbour was always a bit vague about this and it wasn't our business but we did try to work with her re living next door to each other.
Her son loved to be in the garden, would be there all seasons as long as it wasn't raining, and would be shouting, screaming, singing, chanting for hours. He also threw all kinds of things into our garden, broke flowers, (DH is a keen gardener) routinely threw mud at our ancient terrier when he was just lying there sun bathing. We tried to agree various ways forward but none worked as he had meltdowns if not let into the garden when he wanted to be there or was asked to come in when he didn't want to.
We put up a higher fence, we could still hear him through it and it got worse in a way as he then started to throw things at the fence and broke it on more than one occasion which we had to pay for as it was our fence. The neighbours on the other side of them had the same issue.
We tried to be patient, kind, understanding, fore bearing but it wore us down and although this sounds harsh, at the end of the day, he wasn't our son and we wanted to enjoy our garden. We moved, had to declare all this as part of the searches, did find buyers, they even witnessed his behaviour when visiting the house. They came twice, once was on a school day, the other he was at home and in full flow in the garden but as it was so full on, they thought it must be a one-off and bought our house, which was lucky for us but I heard from our other neighbours who we kept in touch with that they were very unhappy and a lot less accommodating than we were.
However, we never kept any of the articles thrown over, always returned them so I don't condone the neighbours behaviour in the OP . Mind you, we had such odd things thrown over, we were never sure if they were 'precious' items or not, so used to return them which also, at times, used to upset our neighbour as she thought we were proving a point. We started returning everything as we got told off when we didn't return a pebble as it was special to him because it was painted - it was one of those peoples people paint and leave in places - but when we invited the neighbour in to find it which she did, it was so faded you really couldn't tell it was once apparently painted like a ladybird