you have to hold the shaft with a allen wrench and then you can undo the nut
i made my own wrench for this to reach into the pulley recess and get the allen wrench down the middle
you could use a big socket and a pair of channel lock pliers to hold the socket on the outside and reach into the hole in the end of the socket with the allen wrench
If it gets to be a huge hassle ( without air it probably will ) you can probably take the alt to NAPA/AutoZone/Advance Auto Parts and have them change it for free or cheap