Stopping customers from leaving without paying

Has anyone figured out how to stop the customers from leaving without paying? I have the handcuffs and they stop the money thief. Nothing seems to stop the customers from leaving without paying. So frustrating! I can't beat level 10 because so many people leave without paying. Help!

I just bought the game and have the same question! What's the point of having the whistle blow if you can't click on the person to collect the money? I wish we could click on say the mustard and click multiple hot dogs and apply it as well. It takes so long to keep clicking for each hot dog. Any ideas?