To block application traffic on specific ports from entering a local area network (LAN), the most effective device is a firewall. A firewall serves as a security gatekeeper between your internal network and the outside world. It monitors incoming and outgoing network traffic based on predetermined security rules and can block specific traffic deemed unsafe or unauthorized. In this scenario, if the administrator wishes to prevent certain application traffic from entering the network, configuring the firewall to deny traffic coming through those specific ports would be an effective solution. This configuration helps in protecting the network against potential intrusions or harmful traffic that could exploit open ports.

References: Firewalls are covered extensively in CompTIA IT Fundamentals and other IT security documentation. These resources detail how firewalls can be configured to block specific types of traffic, a fundamental aspect of network security.