To the best of my knowledge the circuit board in the GFCI won't work if there isn't a neutral, since the circuit runs on 120V. It is the same part that the single pole breakers use. (To test it out, disconnect the white pigtail and press the test button...) I don't remember any code provision that allows the use of a GEC for a neutral, even if it is for .001 watt... Besides, what about future accessories to the tub that do require 120V service?
IMHO, pull the white and there are no questions.
[This message has been edited by Mike Wescoatt (edited 11-29-2005).]