While I am not familiar with US law specifically, the laws in this area are much the same in Europe. Basically, a business can decline to do business with anyone but they can not decline the business for any reason.
For example, I can choose to deny a black person the right to shop at my store but I can not consistently deny all black people for shopping there as it would indeed infringe upon their civil rights.
In reality it might actually be possible to deny all black people on a case by case basis but this would quickly become transparent and likely make me a prime target for civil rights lawyers.