I can't really say what the issue might be without more information. I'll take a guess and say that whatever skill you are trying to unlearn has skills deeper in the tree that are dependent on those points which is why you can't unlearn. If you don't think this to be the case then please let me know which gladiator you are talking about and what skill(s) you are trying to unlearn so I have a better understanding of the situation.
Good luck in the Pit!
-Nate-