I've fired clients. It was usually along the lines of, "If you're interested in the least expensive (fill in the blank), then I recommend that you go elsewhere. Or "I think there are other designers more compatible with your sensitivities".
The most memorable firing I witnessed was by a true artist, George Nakashima, acclaimed Japanese-American woodworker/furniture maker and a father of the American Craft Movement. I heard him refuse a commission from a rather pushy, yuppie woman, telling her that she "didn't understand the spirit of the wood" that she was insisting be used for a table, which George apparently thought ontologically unsuitable. Or else, he just didn't like her and this was his way of making sure he didn't have to interact with her again. It wasn't like he needed her business. If you wanted George to make you a chair, you put three large down and waited 3 years.
BTW, he hand crafted his entire home, never using a nail. It was all pegs and butterfly joints. There was a small stream with a foot bridge, that separated his studio from the family living area.
It was one of my lasting regrets that my wife and I didn't commission a work from him.