You still have to pay the 15% tip which is automatically added on each time you sign for a drink. What you can opt out of is the "per day" tip rate which is divided between the restaurant waiters, cabin stewards, etc. Just visit reception and let them know if that is what you want to do.
Just go to the Purses desk when you get on board and
ask for a form.Then you can pay your own tips at the
end of your voyage.