Select the COURSE SETTINGS button
Click on box next to Create a Grade Book Entry to un-check it.
SAVE the new settings
Delete the column in the Grade Book.
Select MANAGED GRADES
Click in the box next to the name of the column you want to delete.
Then click DELETE
Don't worry! You can turn on the 'Create Grade Book Entry' at any time and it will add the grade column back into your gradebook.