How to avoid client fraud

I have a business idea where I'm gonna help my clients to find their business needs within a period of one month. It's kind of consulting service that I provide to them.

In order to start the service, I require them to deposit $X amount in advance. And, let's say within a period of 30 days, they don't find my service helpful, they can ask for a refund.

Suppose that when I was trying to show a business idea to one of the clients, the client doesn't give any indication whether she "likes" or
"dislikes" the idea. At the end of the month, she just says that she
prefers none of the ideas and she wants to get the money back. However, since there is no tangible product that I can keep, she can use the ideas privately.

How can I avoid this type of problem to occur?