From Der Spiegel:
"So, what will happen when a country is proven to have violated a ban on nuclear weapons? The biological and chemical weapons treaties give the UN Security Council the power to hold violators accountable. The NPT does not include such a provision, but signatories can call on the IAEA Board of Governors which in turn can go to the UN Security Council. Under its authority, sanctions can be applied or even military measures taken. The bottom-line question though is whether the Security Council would actually be prepared to act in the event of an obstinate offender."
I think that's the fundamental problem: The U.N. is wonderful at making grand pronouncements but it is structurally unable to act.