What engine?.....Does it have a starter with a gear?...If so, there is a flywheel with teeth on it under a cover. Hold a big screw driver in on these teeth and against the engine block, while someone else loosens the bolt.
With an impact gun, the vibration loosens the bolt.
Sometimes when working on these things, and we don't have all the amenities we would like, we often have to resort to a little ingenuity.
Impact is impact and we don't care where it comes from. So you get the neighbor kid to hold the deep socket and breaker bar etc. on the cap screw head while you give the wrench a whack (impact) with a 2x4 about 2 ft. long. That bolt will be loose before the flywheel has time to think about moving. Do flywheels have a brain?
There should be some sort of tensioner/idler pully. Remove the spark plug wire from the spark plug first then release the tension on the pully either by moving it with a wrench or a pry bar or most of the time it's moved by the clutch pedal. Once tension is released from the belt you should be able to move the belt enough to get it to slide off the trans pulley. You may need to turn the trans pulley by hand and it will pop right off. Then remove it from the engine pulley. Reverse steps to put it back on. If you have to remove the pulley from the engine and it is a double pully you can sometimes get a pipe wrench between the pullies or use an old belt in the groove of the pulley clamped with a pair of vise grips then loop the remaining part of the belt around something rigid. It will create friction and you should be able to loosen the bolt. Do make sure when you are doing any of these steps you remove the spark plug wire from the spark plug.
Mosrt tractors can have the belt replaced without pulley removal,you may need to remove some guide pins or bolts that keep the belt in the pulley grooves though..dont give in to the temptation to simply bend them out of the way and hammer them back into position,it doesn't always work!..
I suppose you could use a strap wrench to keep the pulley from turning,but you'll need 3 hands to hold it still AND try loosening the pulley bolt..I have had to resort to drilling a hole in the pulley and used a punch shoved through the hole and butted against something on the chassis to hold the pulley still while removing that bolt,when I had no access to an impact wrench or air..I've used the breaker bar and 5 lb. hammer trick Maybar suggested many times with success too..sometimes they used locktite on those bolts and they can be a real bear to get off..I've had to heat a few up and ended up ruining the main seal on the engines crank in the process..for that reason I'd do my best to avoid removing that pulley unless you have no choice or its junk and has to be replaced..borrow an impact gun or take it to a gas station or something,it'll save a lot of grief..
Yeah, TH is right and many a time that pully will give you the same trouble as the bolt did and you may destroy the pully trying to get it off. As mentioned you should be able to replace the belt without removing the pully, avoid having to pull the pully like the plague.