You can do /blacklist (add/remove/list)
/blacklist add - Adds whatever item your holding to the blacklist
/blacklist remove - Removes whatever item your holding from the blacklist
/blacklist list - Lists all the blacklisted items
/blacklist clear - Clears the blacklist of all items
This does stop people duping items in shulkers!

Please be sure to leave my credits in the skript and make sure to give a fair review!

Dependencies:
Skript