Here is a short video which documents a few 'traps' you can setup around your house to stave off a ninja attack. The first thing they list is that ninjas can pick your lock so you could hook up jumper cables to the inside to ward them off. Second up if you set crossword puzzles around the house (according to to video) ninjas can't help but stop and attempt to solve them. Third, the video claims ninjas are incredibly stupid and won't be able to see through simple disguises such as a wig+sunglasses. Lastly, a simple 'no ninjas' sign will keep them away. Right. Okay then.