There are some customers that, no matter what you do, are not going to turn into a done deal. Yet, many people still invest hours of time with them in hopes that maybe it will pan out. Unless you are into doing pro bono work, one of the best things you can do for difficult candidates and clients is to lay them off so you both can get on with your lives. The time you free up can be spent assisting those who really want and need your help.
Your time is tremendously valuable as is the service you provide, so work only with people who are responsive to your process. When you respect, protect, and value your own efforts others will be much more likely to do the same.