Prepaying your gratuities are not gratuities added to your account - prepaid gratuities are added to your cruise cost and paid with your final payment. From what I have read on other boards, prepaid gratuities cannot be removed since they have been paid before the cruise and are not part of your onboard account.
However, you can remove the auto gratuities that are added to your account (if you don't prepay your gratuities before the cruise).
Millennium-Transatlantic-9/01, 10/03, 11/05
Zenith-Special Caribbean 11/06
Back to back Millennium 28 days - Mediterranean 11/18/07, Transatlantic - 12/2/07