Front drive belt slipped off, how do I put it back on?

I have an MTD Yard Machines riding mower that stopped moving all the sudden while in gear. I checked the belts and the front drive belt slipped off the pulley. I can't slip it back on while the clutch is pressed because the belt guards (not the pins) prevent that without tearing up the belt. I've tried removing the pulley but it just turns when I try loosening the pulley's bolt. How do I lock the pulley so I can loosen the bolt?