You would need to dupicate your video track. On the track that is below, you put the blur filter or whatever filter you like (pixelate etc) and on the track that's above you put a cookie cutter and you can isolate the person's face and keyframe the movement.
Hope this helps,
Make a copy of the video on another track. Blur one copy and use the cookie cutter on the other copy to make a cut out around the person's face. Use key frame to keep blur over the person's face as they move around.