It could be some kind of oath. For example, by becoming True Warrior of Tempus, you forsake the use of all ranged weapons. It's fine if you used some before you became a True Warrior; it's fine if you trained skills like bows. But when you become a True Warrior, you swear not to use them again.Kregor wrote:ICly it would be kind of hard to explain how you could forget how to grip your weapon anyway.
Now, since it's not possible to make sure that you won't use them anymore, the skill could simply be set back down to 0. Note that it would not prevent you from training it back up afterwards unfortunately, but if you do that and go against your oath, expect to be thrown out of the prestige class and so on.
I don't think that requiring that someone has not trained a skill is a valid requirement for a prestige class, for two reasons. First, if character A can do all the things that character B can do (that is, if A knows all the skills that B knows), if B is allowed to join a prestige class, then so should A be. In other words, if you meet the requirements and even more than the requirements, you should still be a valid candidate (only positive requirements). Second, this would just mean that the character has not trained that skill when he joins the prestige class; he could still train it afterwards. Therefore, just setting the skill level down to 0 is most likely a much better solution.