I think i mentioned this once on another post here. But i had the idea of, say, once every 10 years or so, the UN gathers the leaders of EVERY COUNTRY ON THE PLANET. They take them to the desert and allow them to view, collectively, a ~50 megaton nuclear detonation (hell after seeing the videos below, 15 would be MORE than enough), and the complete devistation it creates. After they have each changed into a fresh pair of underpants, I'm pretty sure they would all reconsider their eagerness to obtain or use such a weapon.
Edit: Here's a very small video of a 15 megaton explosion, with description of what is going on (size of fireball, time elapsed, etc).
http://www.cddc.vt.edu/host/atomic/atmosphr/bravo1a.mov
(caption under movie:The Castle BRAVO test on March 1, 1954, yielded 15 megatons, the largest nuclear weapon ever detonated by the United States. By accident the inhabited atolls of Rongelap, Rongerik and Utirik were contaminated with fallout, as was the Japanese fishing trawler Fukuryu Maru or Lucky Dragon. The controversy over fallout that simmered around the Nevada Test Site erupted into international alarm.)
From this site:
http://www.cddc.vt.edu/host/atomic/atmosphr/
Can see more videos of smaller nukes, even some of the blastwaves rolling over US troops in trenches (wtf).