If you prepaid it during the booking process, you are probably stuck with it. If you didn't prepay it then it will be added to your onboard account at the rate of $11.50 pp per day and you will have to go to the pursers desk and have it removed, they argue it sometimes and may even tell you that it can't be done, but it can be done, it just requires persistence. Once it is removed you will then be responsible for all the tipping except for drinks which automatically gets 15% added to each bill. Reality wise it is really easy to just let the auto tip go and not worry about it, only thing would be is if you wanted to give someone a little extra.
The $11.50 per day meets the standard tipping norms that are expected on a cruise ship.