If you have a friend on your world and you want to annoy them this is the troll for you. 

If you type in chat /give username dirt 2304 and that will fill their entire inventory with dirt.  But don't worry they will still have all the items they have with them just the rest of the dirt will fall on the ground.

but if you want to get a little more dirty then type this in your chat

/replaceitem entity username slot.inventory 0 dirt 64

If you do that it will destroy the other item but only for one slot. But you can change the spot replaced by changing the 0 to a different number.