Selectively blur many but not all of the faces in a video Shooting videos of Ninjutsu events. Top man insists that there are privacy issues as some of the participants don't want their faces to appear in the resulting video. Would really love to be able to solve this problem because I'm sitting on tons of video that I want to distribute!

Blurring all the faces in the video is not a good solution because first there are the instructors in the the video and second some of the participants would LOVE to see their faces in the video. 

So what I was thinking was to blur the faces of those participants who request that their faces be hidden (i.e. opt-out). This means I'd have to be able to tell the software that "this is the face of Dick, Joe and Harry" and have the software identify and blur all occurrences of those faces in the multiple clips of the event. 

A less desirable alternative would be to somehow pixelate/blurr/remove everyone EXCEPT for the instructors (i.e. opt-in) 

So, my question is:
Do you know a solution to preserve the privacy of some of the participants in a video? 

This is a hobby, so preferably the solution would be inexpensive or even free ;)

I'm totally pessimistic - can you surprise me? :)

Update: I ran a test. I figured Google would have the best technology, because of Google Street View. So I uploaded a sample to YouTube and selected to blur all faces. This is a much easier problem than my problem description but the results were very disappointing. First, it was very disconcerting to see the blurred faces but even worse, the blurring was very "temperamental" - sometimes faces were blurred, sometimes they weren't.

So, I figure that if Google can't do it, then  it isn't doable.

However, and here's the moral of the story: Returned to "Top Man" and mailed him that it can't be done. Return mail was a very surprised "who told you this was an issue"!?! So, the moral of the story is: "Always recheck again and again with the client and re-verify the requirements".

As if we didn't know :)

I'm leaving this question open for a few more days, to solicit answers, and then I'll award the "solution" to the best answer. Thanks for helping!
While its technically possible to have a program detect specific faces and apply a blur on that face. That would require a lot of programming work on your side, there is no free tool or low cost tool that does this automatically for you. This is possible with OpenCV but I highly doubt that this is a viable solution for you in any way, it's a quite complicated task and the result will not be 100% reliable.

The general workflow of blurring out faces would be to semi automatically track the motion of the faces with a 2D tracker in each clip an apply a blur mask to them.
The usual go to software for that is Adobe After Effects or other Motion Graphics video applications like Apple Motion.

Both are no free applications and I never heared of free software that implents 2D tracking together with a video editor. Though there are some really cheap amateur video editing suites out there that even implent face recognition but they all work pretty bad and unreliable without giving you much or no control over whats happening.

For example: http://www.wondershare.com/video-editing-tips/change-face.html
I highly discourage using such software for any work that goes beyond your average random youtube video.

While Adobe After Effects was always unaffordable for hobbyists, Adobe now switched to a subscription based model which makes it very affordable for small projects. You can use the program for free for 30 days (no commercial use) or a monthly subscription for ~30$ depending on the country you live in.

I recommend After Effects because there are ton of beginner tutorials out there for exactly what you want to do and the tracking workflow is quite straight forward. There is also Apple Motion like already mentioned and Nuke and quite a few other applications though After Effects is available for both Windows and Mac and is the easiest to grasp in my opinion.

Edit: I just found out that there is also an automatic face tracker in Adobe Premiere since CS6, this might not yield perfect results but should ease the workflow quite a bit.