Editing help?

The easy answer is Final Cut Express.

For iMovie, there are various plugin packs you can google around for that can probably give you what you want in a limited way.

This company:

http://www.stupendous-software.com/Stupendous/LabelsOverlays/Pages/index.html

Seems to have overlays, like the "dot" overlay that might work for you. The thing is though, I don't think iMovie has any tracking capability, so everytime the subject moves, you may have a heck of a time moving with it.

Also this free plugin pack has an overlay plugin that might work for what you want.

http://mac.softpedia.com/get/Multimedia/FreeShape.shtml

if it's something you think you'll be doing often, or you want precision censoring like blurring or "pixelating", I recommend getting final cut express.

Good luck! Hope this helps