The Move Deleter is the man to see whenever you have a movie that you want one of your Pokemon to forget. This man is able to make a Pokemon forget a move completely at no cost. Also, you can now select the Pokemon that are in your party and the ones that are in your box. This is where you can find the Move Deleter in Pokemon Brilliant Diamond and Shining Pearl.
It will take some time to reach Move Deleter. Get ready to play a good portion of the game first. Along your journey, you will meet an NPC named Cynthia. After she helps you in some areas and you help her deliver her Ancient Enchantment to her grandmother in Celestic Town, you will be tasked with defeating the Gym Leader in Hearthome City. Once completed, you will be assigned the task of going to Canalave City.
To reach Canalave City, you will need the hidden Surf move. Cynthia's grandmother gives you this TM when she talks to her in Celestic Town. You may also want the TM for Fly. This can be found in the Team Galatic Headquarters Warehouse in Veilstone City. Walk or use Fly to return to Jubilife City. From there, travel west to reach Canalave City.
After you reach the city, head north and go to the building behind the PokeMart. Inside you will find the Move Deleter.