If you're lucky, the shaft of the alternator will have an allen key hole in the pulley end, enabling you to hold the shaft (oooh errr) while you tighten the pulley. Otherwise either a chain whip on the pulley (I would only do this if the pulley is steel, use an old belt to protect the pulley), an air ratchet/impact gun, or in extreme circumstances a CAREFULLY placed screwdriver in the fan against the case of the alternator while still holding the fan by hand to reduce the load on the screwdriver. Be prepared to bend back the fan blades though if you do this.
