I wouldn't put duct tape over a webcam lens. Duct tape often leaves a residue. A post it note, with the glue part adhering to the camera just above, below, or to the side of the lens, and with the non-glue part covering the lens would be much less risky.

I think electricians tape would serve admirably. It has to be left for years to leave sticky residue.

O..O
=o=