Right-clicking the Null Charm on air opens a GUI with 5 slots. Placing an item into one of the slots marks it for destruction. Once specified, all items of that type are destroyed immediately on pick-up. Partial stack(s) in the player's hotbar are not refilled prior to destruction. Multiple Null Charms will coexist happily in a player's inventory.
As of version 1.8.1, shift-clicking items into the Null Charm will result in a client freeze.