That can only be accomplished in Grimrock 1 using the Champion:setClass() method to reset all their skills to 0. However, this won't remove their bonuses from those skills, and will also reset their level and experience to 0 (without resetting their health and energy to their original values either). There is code to solve
some of these problems
here, but it does not deal with the issue of losing experience, which is probably the hardest part to deal with.