Basically you'd apply a heuristic that because faces are so special to the human visual system, the perceived energy of a face is higher than the pixels would otherwise indicate.
This would make seams avoid altering any faces in the scene.
Great intuition! The original paper actually goes into this, and they come up with that solution.
This solution is a special case of allowing the user to apply positive and negative penalties as they wish. The latter allows targeted object removal.
One part I didn't get into is artificial weights. The original paper discusses "painting" certain regions of the image as high energy, driving seams away from those regions. This ties into faces when the paper suggests face detection to automatically paint faces with high energy, avoiding these issues in the first place!