How do you get Pokemon off national pokedex on diamond?

To remove a Pokémon from the National Pokédex in Pokémon Diamond, follow these steps:

Select the "Pokédex" option from the main menu.

Press the "X" button to access the options menu.

Select "Remove from Pokédex" and choose the Pokémon you want to remove.

Confirm your selection by selecting "Yes."

Note: Removing a Pokémon from the National Pokédex does not delete it from your game. The Pokémon will still be accessible in your boxes or party and can be re-added to the National Pokédex at any time.