It can probably be refused -- because being Hitler is not a protected class.  However, if the baker said "No, because I don't bake cakes for Germans," that would be illegal.  Bakers can turn down any business, practically speaking, if they are careful about it: "I'm all booked with weddings for that weekend, sorry" is ok; "No, I don't bake cakes for women/gays/Christians/Asians, get outta here" is not ok.