Controlling a mechanical shutter with an electromagnet
You could use a servo with a linkage arm to close the shutter. I'm looking at doing this right now, except I need to precisely control the aperture. A servo will give you (relatively) precise position control, and it's fairly easy to interface with.

If you need to close the shutter very rapidly, then your best bet might be a solenoid. I wouldn't, however, recommend building your own. There's plenty out there that'll do the trick and probably work better than you can design - it'll also save you time. You'll need to use a spring to restore the shutter to the open position. You might try searching Digikey.com to find some.