Not a good way to keep customers. If they want a certain design, they pay you to do it. The golden rule is that whoever has the gold makes the rules. If you want their money, give them what THEY want. The bottom line is that the customer isn't always right, but they're always the customer and that's where the money comes to stay in business. I recommend against a lot of things, but if someone insists on something I've recommended against and they understand that, I design what they ask for to the best of the limits built into web design.
Had one client who insisted on a movie-type presentation with her site. She didn't want to pay for anything more than a few static pictures, though. That's a different story. The contract should state what you will provide, what the design will be IN DETAIL, what the approximate time will be, what the cost will be (plus a possible percentage for going over the time estimated). Get them to sign on the dotted line and you have a contract. Get a deposit (usually around 50%) and show them fairly complete samples of what you're doing.
If I were to come to you to put together a web site that I wanted, and you insisted on doing what YOU wanted, and you expected me to pay for that, I'd go somewhere else and make sure anyone I knew who wanted web design done knew that you would do what YOU wanted rather than what they wanted.
Of course, there are people who want things that just aren't feasible on the Internet, like movie presentations that will take multi-megs of space that anyone with a bad 56K connection can see without a problem. Telling someone that their vision for their website won't practically work for certain segments of viewers is one thing. Telling them you won't do it if they're willing to pay the money is another thing.
If it's a case of nitpicking and revising things over and over, the contract needs to spell out in detail what the design will be. If they want certain colors, they need to pick RGB or Pantone numbers for you to use, and then pay extra if they decide something different.
One client at a place where I worked stated that they wanted a graphic of a rotating car on the main page. That provided, they decided they wanted something more "sporty", but in 2 months refused to give more detail. An SUV was added, and further complaints came in, but still no detail of what make or model of a car they wanted. They knew what they DIDN'T want, but not what they DID want. Dealing with that company changed the way contracts were written after that.
But the bottom line is that you're hired to provide a product to the customer specs. If you can't do it, say so. If you WON'T do it, then you're cutting your own throat.
Lee