All of the keys on the model M keyboard are intended to be removeable for cleaning or for service.
See the following pics on how to remove the space bar with a flat-blade tool and how to correctly re-seat it on the keyboard base. Be careful when you expose the metal spring that you do not pull, bend or deform the spring when reattaching the key stem/space bar.
For the space bar, there is a metal stabilzer bar which attaches to the bottom of the plastic space bar and fits into two slots on the keyboard base. This metal stabilzer bar allows the user to press the space bar along any point on the length of the keyboard, while still having the force equally transmitted along the spring.
http://www.clickykeyboard.com/2009/spacebarfix/001.JPG
http://www.clickykeyboard.com/2009/spacebarfix/002.JPG
http://www.clickykeyboard.com/2009/spacebarfix/003.JPG
http://www.clickykeyboard.com/2009/spacebarfix/004.JPG
http://www.clickykeyboard.com/2009/spacebarfix/005.JPG
http://www.clickykeyboard.com/2009/spacebarfix/006.JPG
I recently received a model M keyboard, but the space bar doesn't work. It appears to be permanently stuck in the down position. How can I fix it?