http://www.bitboost.com/pawsense/
Not tried it myself...and it's not free

I solve the problem by having a keyboard tray, which can be quickly retracted when you leave the room (not much use for a laptop, I know).
Also, if you have a password set on your account you can make it mandatory to be entered on unlocking. What OS are you using?
Other alternative - set your laptop so that when you close the lid, nothing happens. You can then close it when you leave the room.