How do you make a spot shadow on a person in a video?

Not included with iMovie so you need third party software to blur the video.

If you are using iMovie HD 06 there is a low-cost plug-in from GeeThree designed to do exactly that.

GeeThree's Slick vol 6 has a blurring effect, called HiLite Obscure:

http://www.geethree.com/slick/index.html

You can get iMovie HD 06 here.

http://www.amazon.com/Apple-iLife-Mac-DVD-VERSION/dp/B0007LW22G/ref=sr_1_1?ie=UT F8&qid=1425604986&sr=8-1&keywords=ilife+06

follow advice I posted in the User Tip section of this board:

How to create a pointer or a custom title

Super easy method:

I just use the "Titles" in iMovie. Easiest way is to use an "Upper" title and add it to your clip. Type a big capital "O" as your title. Then hit return and spacebar as needed to position the "O" over the player you want to highlight. Takes a minimal amount of trial and error. You can control the duration the title is in your clip and therefore how long the "O" is on the screen.