This problem can be hard to overcome if it is a personality trait. Some people are simply overly familiar with their customers. Relationships develop over time, not in the first few seconds of an encounter.
I visited a coffee shop near my office a while back and the woman behind the counter asked me where I worked. When I told her, she announced that she didn't like on of the women who worked in my business. She raved on for a few minutes until I stopped her, telling her politely that she was talking about my wife. I then walked out, never to return. Such a silly thing to do. She is entitled to her opinions, of course, but if she starts talking about someone from my office, there is a good chance that they are someone I like and respect.
Another example of this is those people who call you and try to sell you something. The conversation starts with no introduction other than a few questions about how your day is going. As they may be a customer you answer them courteously, but the point to the call seems a long way off and finally, when prompted, they tell you who they are and what they are trying to sell you.
Some sales representative are like this as well. You meet them once and then they act as if they are your best friend. Over-familiarity can vary from annoying to downright scary. Your customers should set the familiarity boundary and this should always be honoured and respected.
I used to dine at a particular restaurant on a fairly regular basis. They offered good food, at good prices, and the atmosphere was excellent for conducting business meetings over a meal. Over time, the staff, who were normally excellent, started to become far too familiar at inappropriate times. One time, I was discussing a new project with a prospective client. There were papers all over the table and we were talking quite intensely when the head waiter came over with a glass of wine. He sat down at our table and began to complain about how bad business was. What was worse, he wouldn't leave. My prospective client was clearly perturbed by what was going on and kept looking at his watch. It was a very awkward situation. I finally had to ask the waiter to leave us as we had to talk business. He jumped up, slammed the chair into the table and stormed off in a huff. He virtually threw our meals at us and refused to acknowledge me as we left. The meeting was a disaster.
I understand that this waiter was probably only trying to be friendly, but he overstepped the boundaries in too many ways. As I was a regular, he assumed that we were becoming friends. At any other time, if I was by myself, I would gladly have had a chat. His actions during this meal, though, were completely inappropriate and I didn't go back to the restaurant for years.
I suggest that you think back to the times when you found yourself in a situation where a stranger was basically being overly familiar and how it made you feel. Then make certain that no one in your business falls into the same trap.
Monitor your staff and make sure that the boundaries aren't overstepped, particularly in a male-female encounter. There is nothing worse than a woman going into a shop and having to put up with male attendants checking her out or chatting her up. This is way out of line and absolutely no way to interact with a customer.
It's great to develop a friendly relationship with your customers, but there definitely have to be boundaries. I explain this to my staff, especially if they are young and still feeling their way in the corporate world.
Some people have a problem with the server-customer relationship. They either feel inferior or they dislike serving people. Be on the lookout for this kind of attitude, as it can do your business a lot of damage. Find people who enjoy serving and hang on them. They are worth their weight in gold.