It is not possible to lock specific GUI's (I have looked into it before) but it may be interesting/useful to create some sort of lock that forces a player to meet the given requirements to move an item in a GUI (would have to handle the multiple forms of clicking such as shift clicking or dragging). This could be useful so that people can lock allowing items to be processed. Granted this won't stop them from dropping them on hoppers or the like but still may be a useful addition.