That big nut is a RH thread so you loosen it CCW... lefty-loosey righty-tighty. Run down to your local Home Depot or Lowes & get a big strap wrench to hold the pulley still while you loosen the big nut.
Most strap wrenches are too small for that pulley so make sure to get the biggest size you can find. The last time I looked for one, the tool department didn't have a big enough size strap wrench, I had to go to the plumbing department to find one that was big enough.
They often come in a set like this, only the larger size will work.