That being said, "tu n'en as plus besoin" does seem like a perfect translation of "you no longer need them"...Do not eat food that is too salty, you no longer need them.
Do you really mean "no longer" in English? Did this person need "sweets and salty/fatty foods" at one time? "No longer" implies that once this person did need to "snack on sweets or salty/fatty foods."If you are peckish do not snack sweets, or salty/fatty foods, you no longer need them.