Seems that there may be some misunderstanding of pre-paid and automatically added on your account.
It's as Snooze said--if you pre-pay, it's paid along with your cruise fare and has to be handled out of the H.O. in Miami. If you try to add / deduct after you have pre-paid,- it creates a big problem. Just do the automatic tipping--they will add it to your on-board account. Then if you wish to adjust it, either up or down, go to the Pursers desk, get a form , complete same and return it to the Pursers desk..
ED Note: I restored your post Ron as you mention the forms. I was going to mention them, then changed my mind. Your correct. I do agree with CCL about have to write a reason for your changes.