You cannot keep any of the stuff thrown over but you do not need to return immediately, think you know this. I had neighbours who had kids who with their friends caused chaos. Warm weather - I had back door open and with cuppa in hand was making way to sit elsewhere in house. Football came flying over fence basically arriving in door knocking cup out of my hand. Next event, upstairs bedroom window open, was working in room, decided too cold, stood to close window and split second timing ball hit window just as closing. All the kids scarpered out their gate as they knew I would be annoyed. Went next door to complain and told mother if she did not stop those incidents I would report as being harrassment due to my age. No ball games from then on. Think you may be able to use this in your situation. Would contact landlord with pictures as evidence and tell him you are being harrassed and unable to enjoy your own home. You will find a solution, don't give up as difficult as it may seem.