Whats the best way to hold it from turning? Wouldn't putting it in gear stop it? Perhaps a strap to the frame?

I did wedge a crowbar in there before but it damaged the timing cover and got stuck. I belt is frayed on the side, and I think the belt is making the noise(not belt slipping noise though) not my alt.

 

I always put it in gear then had my dad or sister hold the brake while I do it.

 

putting it in gear and holding the brakes works good for a 5 speed, for an auto you can use an impact. but, you are not supposed to go more than 100ft lbs of torque.