Adult terminals might be unfiltered. Requirement for presence of adult if under a certain age. Monitoring of web sessions to ensure security. In order to protect the library's equipment from tampering, some libraries have and use the ability to monitor various sessions anonymously. Stating this may well have a limiting effect on the behavior of minors.
True enforcement consists of consequences for misbehavior.
Reporting to police of criminal activity. But this confidentiality does not permit patrons to beat their children in the library, shoot people, rob other patrons, or otherwise violate the law. Prosecution is a logical consequence of criminal activity. Online sources of acceptable use policies.