What can the United Nations do to solve the threat of nuclear weapons proliferation?

If the majority of the members of the United Nations or the majority of nations that sit on the UN's Security Council believe that another nation acquiring nuclear weapons is a serious threat to world peace, economic sanctions can be set against such a nation. The UN can also send in inspectors to determine if a nation is becoming a threat to world peace because of it building a nuclear weapons system. If all the members of the UN's Security Council vote to sanction trade or funds of a threatening nation, that sends a powerful message to the offending nation.