As part of the Unified Community Platform project, this wiki and 38 others will be migrated to the new platform in the next few weeks. Minimal changes are expected as part of this migration. Read more here.
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.