Legally, I have no idea if you're allowed to do that or not.

As far as I am aware there has always been custom/law that providers of services may restrict their use. An example would be a landlord refusing to serve a drunkard or ejecting someone who was causing a disturbance in his pub.
My default assumption is that if goods or services are offered for sale then that is dependent upon the purchaser. Shops sometimes refuse to sell eggs to teenagers on halloween because they think that they might use them to cause a nuisance, and a restaurant might refuse admittance to someone who was not properly dressed even without any sign stating a dress code. So, if I went to a local community centre advertising a room for hire, and asked to hire it for my boxing and knife combat class, then though I would think them small-minded bigots for turning me away I would not think that they should be forced to take my money just because I was the first to apply for the vacant slot.

my primary reaction was sadness because people still react like this;

It is indeed most unfortunate that people do adhere to these strange superstitions.

By the way, I am not getting comment notifications from LJ today, so if I don't reply then it's because I haven't spotted your response.