I got this one time at IHOP. I asked the manager what the SOP was, and she told me to void out everything on there bill, take the cash they had, and tell them they owe the next guy the money they owe me. Next time they came in I reminded them that they owed me $5 and refused to let them in the store until they paid me, and showed that they could pay for there order (meaning, I kicked them out when they showed me less then $15 between the three of them).
I also had a couple walk out on there check. Let me tell you, it's generally a bad idea to do that when you know the server. It's also a bad idea to come back the next week. Again, I refused to let them in the store until they paid me the money they owed me. Which I jacked up because none of there group gave me a tip, and one of them walked off with my pen. They ended up paying me $30 (they owed 23), and walking away because I threatened to call the cops if they didn't pay.