I am no 'alim but I'm pretty sure if necessity arises, it becomes permissible. Hitting the face is not allowed even for punishment's sake since it is not a necessity. But if there is no way to defend yourself but to strike the face, it would be permissible due to necessity. In j-had (on the battlefield), there is no clear ruling on avoiding the face, for example, since it may become a necessity. Read here: http://www.sunniforum.com/forum/show...tting-the-Face