The ones from the previous event in 2011 can be placed in the Armoire, assuming you don't already have the same ones in there. ie the Red, Blue, Green, Solar and Lunar ones. The Striped and the Morning ones however cannot be placed in the retainer. Same with the black ones from the vet rewards. People ran into this with AF as well.

There's just some items the game doesn't let you throw away, and in a way it's a good thing.

What "I" think they could do, is implement an NPC. You tell the NPC you want to trash X item. It checks all your other storage places(Armoire/Retainers) for duplicates and if so just trashes that one item.