The Move Deleter is located in the large, orange-roofed building at the center of Fianga City. He is the old man with the hat and coat in the bottom-right corner. The Move Deleter is also available as a purchasable Secret Base upgrade after the player gets their final tier of upgrades in the Aroma Region.
Use the Move Deleter to make any Pokemon forget any move. This is especially useful if the player taught a Pokémon an HM that is no longer needed, as those moves cannot be replaced normally.