WHAT?! He is watching YOU and not his cats?! Oh, I would be very mad! I have 2 cats, and I do NOT like strays because they attack my cats and Lord only knows what disease they would have. My cats are fixed and strays will spray to mark their territory. How were you to know the cat was a neighbors? I would call the police and have him remove ths sign. That is harassment. I am sorry this happened to you. Good luck!