you can delete all game patches using the button combination method on the system blade.
i am not sure what that combination is, but im sure you could do a search to find it.
edit:
from teamxbox
QUOTE
Go to the system blade and then select memory.
Press Y on the HD symbol and then press X,X,Left Bumper, Right Bumper, X,X. Once you do this a messsage will appear saying: Do you want to perform maintenance on your Xbox 360 storage devices?
Performing this function will also clear any software updates.