Tiger poo or failing having a willing tiger to hand tea tree apparently works as a deterrent or throw more tempting foods into the other garden along with as much of the intruders poo that you can gather,might make them think of they have to deal with it
Or teach Tipsy some kitty kung foo moves-concentrating on the more legal but lethal ones
Sorry FA i know it's a pain for you and Tipsy and i hope you manage to resolve the problem without to much hassle