Place a big furniture piece, such as a tall and wide bookshelf, china cabinet or hutch, in front of the door. Position the furniture so it covers the door completely, obstructing the view.
Hang a curtain rod above the door and attach a curtain that extends beyond either side of the door and drapes to the floor. This method allows for access to the door if necessary.
Span a curtain wire from one side of the wall containing the door to the other, attaching it with eyehooks. Hang from the curtain wire fabric large enough to cover the door and entire wall.
Lean a large picture or mirror tall and wide enough to stretch from one side of the door to the other. You can also hang it directly over the door if possible.