A lockout device for an opening of a collection box, such as a postal collection box. The lockout device can include a cover sized to block the opening to prevent access to the contents of the collection box through the opening. In one example, the cover can be assembled over the opening and a locking pin can be inserted through apertures in the lateral sides of the opening walls and lateral sides of the cover. The locking pin can be lock in place using a lock and lock guard. In another example, the cover includes an internal extension assembly with clamping mechanisms that can engage with the lateral sides of the opening.