the simple answer is yes. the traps are on his property and it cant necessarily be proven that they were intended for your cat, though even if it could, i dont think it would do very much. as evidenced by other answers in this thread, people do not care about animals and are perfectly willing to be cruel to them, even if they are beloved pets. that said, i would still recommend keeping your cat indoors for the safety of the cat and the local ecosystem. and as has been previously pointed out, a neighbor could take your cat to a local shelter, which would not be ideal for you or the cat. tldr; yes he can, and do keep your cat indoors.