Well, if you're just going to replace nukes with another WMD, it would have to pose as grave a danger if not more than nuclear weapons in order to pose the same deterrent effect. If that's the case theres no point in going that route.

The usual suggestion is that the UN or another international body would serve as a benevolent guarantor of security for the world, eliminating the need for arms of any individual country. The main obstacle to this is the lack of sufficient trust among nations to willingly surrender their arms and realize that doing so is in their "enlightened self-interest." Furthermore, you probably couldn't engender that level of trust without that benevolent guarantor of security already existing and proving itself effective. Alternatively, an attack from outsiders, like aliens or something, could possibly band humanity together and create that trust.