If its just in one area of your yard, a motion activated sprinkler would be a good humane solution.

Semi-auto assault rifle... would be an inhumane solution.

A cat eating dog.

Seriously with out a fenced yard with a dog to guard the area, or motion sensor sprayers, you cannot keep cats out. They know no boundaries and only respect fear.

This post was edited by texas-weed on Sat, Apr 6, 13 at 16:35

Use a racoon or skunk live trap to catch the cat. Then after catching it thoroughly soak it with a garden hose before releasing it out of the trap. Most cats catch on fast.