A system for controlling communication-access within a computer network, includes an access-prevention device connected in series with a computer's bi-directional network-communication port for preventing the computer from receiving and/or transmitting any communications from and/or to another computer within the network; and a control device for selectively controlling the access-prevention device. Such an access-prevention device including a control terminal for connection to the control device is disposed within a communication-access control apparatus that may be connected between the computer network and the computer. Alternatively, the access-prevention device is disposed within the computer chassis, the modem, an external network-access terminal or a firewall device. The control device includes a manual switch actuator disposed on a chassis containing the access-prevention device, or a remote-control device, or a computer keyboard or a mouse, or a presence detector, or an inactivity detector.