My library knows that service animals provide vital assistance to patrons with disabilities, and that they must be allowed on library premises without pre-approval.
We also know that when a service animal is brought into the library by a visitor, we can only ask if the animal is needed for a disability, and what work or task it has been trained to provide.
First: Because of the restrictions you cite, continue to not take the bait! If this person is indeed taunting library workers and daring them to deny access, continue to address the situation with tactical restraint.[1]