Good morning,
 
Craftsman model 917-203900 / Serial # 021714A00317 lawn tractor. I am replacing the front two drive belt pulleys. I got one off and replaced but on the second one, the bolt unloosened some and now just the bolt and screw turns..cant unloosen it..cant tighten it back. On the diagram, how can I hold Bolt #185 to keep from turning? I am replacing part #50, front pulley.
 
Thanks for all the help so far.

That bolt is a carriage bolt with a SQUARE bottom to hold it from turning in a square hole:
 
https://www.ereplacementparts.com/bolt-fin-hex-3816-unc-234-p-1057116.html 
 
Simply pushing down on the head as you turn the nut, it should stay.  
 
Worse case scenario, grab it with a vise grip to remove it and replace with a new bolt.
If you can tighten it back up, spray some lube to clean up the threads, it may then come off..
 
,

Thank you
 
Problem is, that bolt is hidden under the frame behind the steering column. A Sears youtube suggest putting a small screwdriver in there to hold the bolt, but I cant make that work..