NCL makes the distinction of calling it a service charge and not tips or gratuities, which they put in a different category.
My understanding is you cannot simply opt out on day 1 of the cruise. You can adjust the amount for specific service problems but they request that you give them a chance to fix the problem. If the fix doesn't meet your approval they have a form you fill out and list how much you want to remove. The form has a blank for 'reason'.