What to do with customers that don't pay their bills?

Although it rarely happens for me occasionally I get a catering customer that doesn't pay their bill. My real question is what can be done? It is really frustrating ,I mean you pay for all the food and your rent on the facility all the linen and then when you give the customer their bill they say they will pay you in the allotted time ( in my case it is net 7days which is better than any restaurant would let you do) . The first week goes by and you don't see them, the second week goes by and you call, they say they will be in right away, then the third week goes by and still nothing so you call again, you say I understand weddings can be expensive perhaps you could manage it in a couple of payments. They say" that would be great I will get cheques to you right away" after a month still you haven't seen them. Now what? It's a small town you think you can trust people but no the ones you don't expect are the ones you have trouble with. I'm thinking that I should run an ad in the local paper "Cognates ____ and ____ on your marriage last month Now please pay your caterer"