Bans will not only not prevent them being developed, probably even by a technologically advanced State that is a signatory to the treaty, but it will also not prevent them being used by rogue or puppet states who don't care about bans, or who use them at the behest of a signatory state that is just using them to do their dirty work.
Any state today is dependent on trade from the international community. If the US and the EU (or any other large fraction of the international community) decide not to trade with a country, and not grant bank transfers to that country, that has a huge effect on their economy. The countries able to withstand this are countable on one hand. Of course, trade sanctions are not a plan, but the lack of a plan.
It is always better though to help the particular country address their actual problems rather than supporting their approach. For example, perceived threats can be thwarted by establishing a neutral buffer zone controlled by a third party.
So no, contrary to the common opinion on Slashdot, I think collectively agreeing to not use a certain, dangerous technology can be useful, and is also enforceable.