Turn on your Nintendo 3DS
On the HOME Menu, highlight Pokémon Omega Ruby
Press and hold A, B, X, and Y at the same time
Keep holding the buttons until the game asks to reset the save data
Confirm the reset when prompted
If you want to delete the save manually, start the game and go to the title screen
Press Up on the D-pad, B, and X at the same time on the title screen
Confirm the deletion when prompted
Start a new game after the save is erased