Attempting to summon in an item with an ID (such as a feather) will prevent the item from being picked up.

For example, try using this command in a command block, players won't be able to pick it up:

/summon Item ~ ~ ~ {Item:{id:minecraft:feather},Age:0,PickupDelay:0,Motion:[0.0,0.7,0.1]}

However, putting in the same command but without the ItemId part

/summon Item ~ ~ ~ {Age:0,PickupDelay:0,Motion:[0.0,0.7,0.1]}

will spawn in a dropped stone block that CAN be picked up.