what is a good way to stop the crank from moving when removing the harmonic balancer? so that when u try to losen the bolts the crank dont spin

 

If you use a breaker bar or something long for leverage and a 6 point socket, you can usually give it a "pop" quickly and they will break loose. If not, slip something through the spokes of the pulley and let it spin until it safely wedges against something.
You could also try a "strap wrench".

 

I just used an impact with the correct socket, so it would only turn the bolts, came off really easy.