/me likes freedom, including freedom of information.. the "wrong" people have all and any information anyway.
you will only need little optical power, my guess would be that 20mw can be damaging in good/bad cases. same problem as with the eye, the lens focuses it very sharply onto the retina/ccd/cmos. many/all b/w cams will see IR.
three problems i see, for someone willing to destroy cams:
-you need to point the dot onto the lens. sounds too clear to even write down? dont forget that the lens on a webcam is up to 5mm small.. bigger objectives/lenses still need to be hit, and in that case with the proper angle too, else the beam is focused somewhere inside the tube.
-you will (normally) only kill single pixels. to "erase" the cam completely that way, you have to move your lasersource. ..would be a funny video, i bet!
i wouldnt say its impossible to kill the chip entirely with one shot, but dont know what damage-mechanism would fit for that. shorting out entire lines perhaps?
-you dont know if its dead, you dont know if you are recorded. but people watching the screen will know immediately that something is going on. robbing a bank? they have a 1 minute advantage now.
to blind it temporarily should be easier and more practical, perhaps use IR leds. several, if you like. a handfull of those, 10W IR output, aspherical lenses, and there you go. insist on lasers? use a pointer as target-laser.
built it portable, and post pictures! ..whoever feels spoken to ;-)
manuel