The best way to get rid of chemical weapons is to make people realize that they are less effective than conventional weapons. Most armies realized it at some point.

Chemical weapons tend to be hard to control, easy to defend against, and not that lethal once you consider the difficulty of dispersing the agent.

Even terrorists don't use them much. The famous Sarin gas attack in Tokyo subway "only" killed 10, a regular bomb would likely have been much worse.

The only application where chemical weapons are used effectively now are in crowd control and less lethal weapons (tear gas).