Lockable covers are used to cover either of two types of shield blocks to prevent access to circuits within either of two types of external ports on computing devices. Each such shield block covers the port contacts associated with these circuits, includes locking surfaces for holding the lockable cover in place, and is itself held in place by the attachment features provided on the external port for holding an external cable in place. One such shield block is held in place by a pair of screws extending into threaded holes in the external port, while the other such shield block is held in place by a pair of flexible wire bails rotatably mounted in the external port.