Some people have used a hot knife - which is basically a wire attached to a battery pack that heats up for cutting. I'm not sure what it will do to the plastizote though. Also, I have seen people buy one of those large trowels - the ones that are used for smoothing concrete/cement - and they sharpen one of the edges to use it to cut the foam.
For myself, I use a belt sander. I trim down the foam to the basic shape I want with one of those disposable utility knives that cost a buck. I use the full blade extended since it is nice and long but usually the blade is dulled by the end of doing just one weapon. Then I use a belt sander to trim/shape it.