$game_party.lose_item(item, n, include_equip)
n = the number you want the party to lose.
include_equip defaults to false. You don't have to include the argument at all, but if you put true in its place, your party can lose equipped items on top of inventory.
item = the actual item you want lost, not an ID. Thus, it should be one of the following:
or
where the x in any of the three is the ID of the item, weapon, or armor to be lost.