If you truly cannot produce what they're asking for, be honest but then help them find someone who can, even if it's your competition. They'll remember that you went the extra mile to make them happy. The last thing your customers want to know is that they rank at the bottom of the food chain. It is acceptable to say that you will need a few days to do the job right, or that you'll knock off a few bucks in exchange for their patience. It is never okay to imply that they aren't as important as all your other customers.
Unemployment is boring.
Try to find something to love about every job or project you do. Otherwise, find a job you love. Life is too short to be bored or boring. If you've been doing something the same way for years, it's a good sign you're doing it the wrong way. Maybe it's time to find a new and better way to do it.