I believe it's been done by changing the law. One of the problems is that the international treaties are sometimes cited as an impediment to doing that. I'm not so sure they are the impediment. The Netherlands, for example, has had a long-standing policy of non-prosecution for possession of small quantities of drugs. So even though the laws remained in place, they used the policy of non-prosecution. We could do the same thing here in Canada. That would be one way to ameliorate the harshness of this law.
On October 18th, 2011. See this statement in context.