Answer:
The only way to get rid of it for real is to remove the insulation that the guano is piled on. Also, you'll need to scrape/disinfect the permanent structures (beams, floor boards, etc) that the insulation was laying on. And, you must wear breathing protection to keep the mold spores out of your lungs.
It's sad but true-- it's an expensive, time consuming process.
My knowledge comes from talking to experts when my MIL had the problem in her attic, gleaning the internet for less expensive/diy solutions and direct visual contact with a "guano"load of guano and the insulation/floor boards the guano had piled up on.