In a former life, I had a business that involved selling pricey items to cutomers who were comfortably well off. For the most part my customers were a joy to deal with. Those that were not, basically ran me like a rented mule and then shopped out my hard work to a lower bidder. I'm not sure why they did that, but I began to take pains to weed out customers who might be a problem. One was tough, in that I really liked him and he always showed a lot of interest, but after close to 4 years of running me around he finally stepped up to the get a piece of work and then tried to hit me for a discount and a long payment schedule. I sold the item to another customer and he got the hint. I still wish I had a found a more diplomatic way of cutting him off, but I was not thinking straight at that time.
So, all that says is that if you think a customer is not working out figure out a kind way, if possible, to sever your relationship. Hard to do good work when your aggravated.