How do I make a person's face unrecognizable?

Question

I edit a medical journal and, though we've only showed images at our site, we're now thinking about putting video up also.
For still images we overlay a black rectangle across a patient's eyes, eyebrows, and enough of the area so that he or she is not recognizable.
Now that we're looking at video, I'm wondering how to do the same with video of a person's face or head and neck. I would like to use a similar overlay when editing video that shows this area of a patient's face. Patients rarely remain in the same place from one frame to the next.
Is there a tool in FCP that will easily do this?

Answer 1

Thank you all for the help. I am glad that there is at least a way to do this. It's not as simple I would have liked, but it's do-able. In addition, I can see that these tools will educate me more about FCP features with which I'd never needed to work.
And, to allay Mr. Cookman's concerns: The authors are physicians who get the prior approval from their patients to use their images and/video for the publication. We simply prefer to disguise the features so they are unrecognizable.
Again, thank you for your kind assistance.