Unfortunately, workers often use too long nails when they think the underside won't be seen—for instance, your basement ceiling and the underside of the roof in your attic. End Nippers are a tool made to clip the nail off flush with the board it is coming through. It takes time to do this job, but the results are clean looking and safe. Wear some safety goggles or a face shield to protect yourself from dropping nail ends. You can get these from most home improvement stores.

Spray on foam is dangerous to apply; you must completely cover your skin and use special masks to avoid breathing in the vaporized chemicals. And while I've read that these foams have a resistance to burning, once they catch fire they release dangerous chemicals. There are fire resistant paints, but I still recommend the end nippers.