$6 Harbor Freight mini hacksaw is my main tool for this purpose. A hose clamp or piece of tape for a guide. I even used a pipe cutter once and it actually worked pretty alright, kinda slow if anything.
+1 on the garden hose clamp + cheap hacksaw. I've cut 5 steerers like that, 4 carbon, 1 alu. I changed the blade once, probably should have changed it more often. The local DYI store sells blades for very cheap.
Spray water to avoid breathing in the carbon dust. You'll be shocked to see how black the water gets. You definitely don't want to breathe that.
Don't use a pipe cutter on a carbon steerer, you may crush and delaminate the carbon.