I summoned an item I used as a display because I am working on a custom Lever model.

I set the Age to: -32768 and the PickupDelay: -1 (Basically makes the item unable to be touched by players)

//Summon item that can't despawn or be picked up
/summon item ~ ~ ~ {Item:{id:"lever",Count:1},Age:-32768,PickupDelay:-1,Tags:[LEV]}

//Summon entity to pickup the item
/summon fox ~ ~ ~

The item is unable to be picked up by any player but when a mob steps over it gets immediately picked up by the mob...