My guess is that if you can deem a ball unplayable at any point but then it is no longer a provisional but your next shot. I found this which seems to support that interepretation: I also think there is a rule which forces golfers to identify their golf ball, you just can not refuse to (maybe I am wrong)

27-1
c. When Provisional Ball to Be Abandoned
If the original ball is neither lost nor out of bounds, the player must abandon the provisional ball and continue playing the original ball. If he makes any further strokes at the provisional ball, he is playing a wrong ball and the provisions of Rule 15 apply.
Note: If a player plays a provisional ball under Rule 27-2a, the strokes made after this Rule has been invoked with a provisional ball abandoned under Rule 27-2c and penalty strokesincurred solely by playing that ball are disregarded