Definitely "in the face" although I can see why most of the voters voted for "on the face". It's because the preposition "on" is often used when talking about something that is related to the surface, such "on the table", "on the floor", "on my skin" and so on.
If you have a question about the English language and would like to ask one of our many English teachers and language experts, please click the button below to let us know: