Was money exchanged? Does your standard contract have language that covers this type of situation. I think there are precedents. I believe a bakery didn't want to make a cake for a gay marriage. I think there was a lawsuit but don't know what the outcome is. If you are facing this dilemma, then you probably need to consult an attorney in your jurisdiction. Or call the ACLU and see if they will advise you, though they might side with the client. Are suspected neo-nazis a protected class when it comes to discrimination. There's just a lot of legal questions that you will probably not get proper advice on from a forum.