If yours doesn't have the hole for the allen key (some non-genuine ones may not), you can hold the pulley tightly by making a wrench from an old fan belt and a piece of tube. You pass a loop of belt through the tube, and around the pulley, then pull the loop at the other end tight and clamp it. There are ways of devising a screw for tightening it, if you have the facilities and inclination to make it into a proper tool, but for a quick one-off you may find that levering the belt tight and clamping with mole grips against the end of the tube is sufficient.
You then just use it much like a chain or strap type oil filter removal tool. As you apply leverage, the tube will deflect to an angle, which actually pulls the belt tighter, so you just hold the tube and apply a spanner. As Dan says, it will be very tight.
You may even get away with just looping the belt round the pulley and clamping it together tight against the pulley in the vice.
I expect that you can buy a tool much as I have described somewhere, with a screw for tensioning it, but I can't find one right now. However, you could probably use a strap wrench (see link below) if the strap is wide enough to span both pulley flanges, or cut a piece of fan belt just long enough to fill the pulley groove and use the strap wrench on top of that.
Before I knew of the belt trick (can't remember where I got that from, maybe I saw a lever type strap wrench for an oil filter), I once clamped one in a Black and Decker workmate, with the pulley in one of the notches in the jaws. It worked, but does have a high risk of damaging the pulley.
http://www.frost.co....-120mm-cap.html
Edit: See link below which suggests using an impact wrench. That way, with sharp impacts, the inertia of the pulley and alternator armature is what the torque is reacting against, so it does not need to be held.
http://www.peachpart...act-wrench.html
Edited by tiger99, 04 October 2013 - 07:35 AM.