By what God given authority do you have to ban an ...

By what God given authority do you have to ban an adult visitor for life from entering your public library because he presented two small well behaved young gentleman with $1.00 each for being so good and quiet while reading their books?
What future self evident freedom of yours, that if I work hard enough to disrupt, would be mine for the taking?
P.S.: The psycho parent who made the complaint allowed the boys to keep the $2.00.