I don't see anything wrong with having a sign that says "please take one", the only problem is that a family that will clean a plate like that probably won't read it. People are going to be jerks if they are going to be jerks. A sign really might not help. But if it makes you feel better, go for it!
The state board of health won't let us sample here unless we have hand washing stations, covers for the food, no cut foods without special precautions. We can sample whole small foods like cherry tomatoes, or individual apples.

Probably best way to handle that,is to keep the plate at the back of the table w/ the product sliced or chunked up in bite sizes with toothpicks in each so when someone shows interest in a certain items,ask "Would you like a sample?"present the plant,holding it the whole time so you can remove it once one sample has been taken.

PP

Its too bad that one person ate so many samples, but I don't feel that people who take samples have any obligation to buy from the vendor. That's why they are "free samples."

Samples are a touchy issue in our area also. We are allowed to put out baked goods, but fresh samples shouldn't be cut into pieces (contamination, etc.) On the other hand, the Ag and Markets guy told me that they don't visit markets unless someone complains.

The debate as to whether to offer samples has many arguments on both sides. If you have customers waiting in lines to purchase your produce then samples are just a waste of valuable table space and time.

Sampling is similar to the credit card issue. I finally droped my service plan with the bank today after 2 years of not accepting a single credit card payment-- never had the time to handle the transactions.

You never want to be rude to the customers. Sometimes it is best to laugh and ask them what they enjoy most about some quality aspect of the produce. I can't tell you how many boxes of produce that were dragged from my tables to the street by a bag of produce, a purse or a hungry toddler with an unexpected reach. There are also the outright thieves who take more than what they pay for. All of these are typical of my favorite customers because they return regularly to give me their business.

I will regularly snap a cucumber to display the texture and inside or cut various melons in half since the 15+ varieties I offer are not ordinary, but I don't offer samples to eat. The senses of their eyes and nose will usually make the decision to buy or not.

Costco does a lot of this sort of sampling, often having 5 or 6 sampling stations with various products. so I took a look to see what they do.
They portion out very small portions, one big bite size. and they only put out very few at a time, sometimes only setting them out on the serving plate one or 2 at a time, as the last one is taken. They prep the samples as they go, so it looks like they only had so few because they just made it, and that gives the impression of being very fresh to. They really have it down to an art form, dealing out only just enough for the number of people that are right there, plus one. So a person may take 2 bites, then there just isn't any more!
Of course, if there is only one person working your stand, and you are taking money and selling, you can't work it that closely. It's one persons' full time attention to keep it going that way and pitch the product.
Hope that helps some.
Oh, and my favorite question... "Would you like one or two?" Either way, the answer is, they want to buy SOME.

I've actually been waiting on this thread to share a few stories from our market. I had been selling quart baskets of tomatoes and couldn't keep them filled fast enough. So, one lady walked behind my table where the tomatoes were being kept and took a basket and filled it up herself. Only her basket was about 3 times higher than what I was selling. She could barely keep them from falling out and then asked me if it looked ok. With a very surprised look on my face, all I could say was Wow, that looks really good.

We also sell varietal honey and give free samples. We like to talk about the floral source and the difference in flavor and color. One lady would always come by every week and ask for a sample. After tasting it, she would make a face and say "I just want regular honey". After a few weeks of this, the next time she came by, we told her that today we just have regular honey. She tasted it and then bought a jar.

So, sometimes tact is a two way street and you have to make a judgment call. If I were in that situation, you bet I would pick up the plate out from under their hands and tell them that I need to refill it. I would then show them a whole canteloupe, tell them the price and ask how many they wanted to buy.