I'd argue that it's not that easy to "ban" something by getting the UN to say so, or even getting conventional treaties signed. First, the nations have to ratify it. And even if they do, they have to abide by it. Here are some random examples that I googled-for off the top of my head where these treaties, laws and the UN have failed.

Torture:

http://www.cfr.org/international-law/united-states-geneva-co...

Tear Gas:

http://www.washingtonpost.com/news/morning-mix/wp/2014/08/14...

Kyoto Protocol:

https://en.wikipedia.org/wiki/Kyoto_Protocol

Extrajudicial Killings in Drone Strikes:

http://www.theguardian.com/world/2012/jun/21/drone-strikes-i...