To ensure that only assigned devices can connect to a switch, the network administrator should enable port security. Port security restricts port access based on MAC addresses, allowing only pre-configured devices to connect to the network. This helps prevent unauthorized devices from gaining access to the network. Other options like configuring ACLs, implementing a captive portal, or disabling unnecessary services serve different security purposes and do not directly restrict physical port access based on device identity.
Reference: CompTIA Network+ Certification Exam Objectives - Network Security section.