The question does not give enough details to give a correct answer. If the restaurant is open to the general public and the KKK member isn't disrupting normal business, the owner can be forced by civil court to serve him. I believe in private property rights, but I understand that when you open your property to the general public to conduct business, you enter a new legal realm.
If the restaurant is open only to member, or if the KKK shows up wearing a KKK hood, the owner has rights to boot him. The term "forced to serve" leaves much to the imagination also. Does it mean the KKK member filing a civil suit? Or does it mean the KKK member using his own physical force to get a meal.
In a more real-life question: If David Dukes walked up to a hotdog vendor on a New York City street, does the vendor have a right to refuse to serve him?
A black principal, in Ann Arbor Michigan, segregated white children so that they could not attend a field trip to see a black rocket scientist. This is a different set of circumstances, but does bring up the same set of question concerning race.