No. Once a cheat is activated in Midnight Club 2, it cannot be toggled off within that save file. The cheat persists until you:
On PC, you might manually edit save files or use a trainer to revert specific memory values, but this is:
Don't use cheats on a save you care about. Create a separate "cheat save" for experimentation. Play the intended challenge for the authentic experience—Midnight Club 2's progression and unlocks are designed to be rewarding.