I am trying to change the drive belt on a YardBug lawnmower. How do I keep the engine from turning while breaking loose the bolt which holds the drive pulley on? (I have a socket wrench set... no impact wrench.)

Hi,
My question would be why, unless you split the deck pulley. Belts can generally be replaced without removing it.
If the need still exists, try removing the blower housing from the top of the engine. Then very carefully wedge the flywheel. This can be a number of ways. The first I might try would be to move the pinion on the starter up into the teeth on the flywheel. Then take a rag folded over once or twice and get it jammed between the teeth of the gears. This should hold everything.
Peace,
Clarke