Most municipalities now have regulations about cats at large. Research this topic with your local city government or call your animal control officer for information. Chances are that you can legally catch cats at large and haul them off to the shelter. The animal control officer may help you with this. Otherwise, you could try several cat-scaring methods. Motion-sensor-triggered contraptions that spray water at animal intruders or emit high-frequency sounds can also keep cats at bay. Repellents work well in many situations. Try a liquid formula that you apply with a hose-end sprayer. Or purchase covered repellent holders that you fill with puffed rice impregnated with repellent oil. The covering protects the repellent from the weather, so it lasts longer.