You don't need a hex wrench to work with the rear wheel spindle. You do need one for the front wheel though.
All you need for the rear is anything big enough to fit in that hex head to prevent it from spinning (a 1/2 inch hex head or large flat head screw driver works too). It's that big nut on the other side that you need a wrench for. Once you undo that nut, you just use a rubber mallet to pound out the spindle. Do the reverse to install. Make sure you note the spacer orientation so you install it the same as before. Also don't forget to clean and regrease the spindle, spacer and drive gears.