If you want to hide the identity of a person / thing / place in a video, then you’ll have to mask it or create a blur effect on the object. Here are a list of links that will help you easily blur a video or blur a part of video in MAC:
Go to this page : http://www.rippletraining.com/free_tutorials_oldies_but_goodies.html and watch the video titled Cops. It is a free video that will give you step by step instructions how you can introduce blurs in a video.
Alternatively, you can download the Mosaic Filter effect, one of the many freely available effects from Mattias and use it as a plugin.
Link for download: http://www.mattias.nu/plugins/
MacMost Now has come up with an excellent video tutorial which will help you achieve Blurring an area in iMovie 08: