if you buy packaged prune juice, they are said to be high in sugar content. so even if they advertised as low calories, healthy choice. the sugar content will still be stored as fats in the body.
unless they are naturual prune juice that you prepared yourself. contradicting as it may sound, most fruits are high in sugar content too, even though they induce bowel movements, it is recommended not to take in too much as you approach the night time. again, these sugar, though they are known as simple sugar, will still get their chances to be stored as 'extras' in your body, ie fats.
detoxing this way is not a good way to lose weight effectively anyway
hope this helps!