Hack saw and the razor blade works, I use a Fiskars multi-purpose shear and cut it down in a spiral till I get the too the back to the reflector. Quick, simple and less dust for the reflector to collect
I usually use a hacksaw to cut the cam off but I leave it a little long. Then I sand it down to the final length. Now I have a disk sander that makes quick work of this, but before I got it I used to set the sandpaper down on a flat surface and run the bottom of the reflector over the paper in circular and figure 8 motions. I found I could get a lot better looking edge this way. Every time I used a razor blade, I always ended up with a wavy cut.
I picked up a couple of 'razor saws' a while back. They are used for cutting plastic. They are very thin, have a very fine tooth and work great, I wouldn't buy one for just one reflector, but they come in handy .