How to disable Phantom?
My sharpshooter just got the Phantom "ability" via the Advanced Warfare Center. This is completely useless (why would I ever scout with a sniper?) and means he can't participate in ambushes.
How do I turn it off or get rid of it altogether?
There is no natural way to remove or suppress this ability.
As you have seen, this can indeed act as a detriment. A strategic way to work around this limitation is to use this character as the one to take the opening shot in an ambush.