The Keep Locked Shut When Not In Use 5424 Sign will recognise the fact that people may need to use a door, but that it must be locked shut at all other times. This may be because the door is a designated fire door.
Have a sign displayed on each of the sides of the door to make sure this rule is enforced. The sign is also ideal for doors to medicine cupboards etc.
Fixing requires anything as simple as sticky pads, 'No-nails' type adhesive or double-sided tape.