Eureka!
Alls I had to do was check the AoS manual.
Pg 9
NOTE: You can reset the game software by pressing and holding down A Button + B button + START + SELECT simultaneously.
On a side note:
Mario Kart Advance's manual states:
To erase all of your saved data, hold the L Button, R button, B button and START when you turn the power ON, then choose "Yes" when the confirmation message appears.
Why can't this sort of thing be hardcoded into the cartridge format itself? That way there'd be a standardized button combo to erase any game.