How do you stop sanctions if you already follow the demands put in front of you? The reason the world isn't on the US side against Iran wholly as it were in the beginning of the sanctions is that Iran did live up to the demands.... and then the demands were changed. The US broke the deal, not the Iranians.
You stop the sanctions by meeting the new demands. The US decided the original deal did not offer inspectors enough leeway to ensure Iran was actually halting nuclear weapons development, and so it added more stringent inspection requirements. Adopting or walking away from a deal is a mutual decision. Yes, the US decided to put new terms on the deal. Iran could have accepted adding real enforcement mechanisms to the deal and ended the sanctions, but decided otherwise.
This thread is being rate limited. The commenter below is incorrect. The post deal demands included restrictions on nuclear-capable ballistic missiles, too, but they also included changes to increase the access of inspectors. This was a substantial part of why the original deal was rejected, the new administration believed the original restricted inspectors to the extent that Iran could still develop nuclear weapons in secrecy.
The existing deal already has enforcement mechanisms under Article 37. If Iran violates the deal, the UNSC sanctions are reinstated by the P5+1. If the P5+1 violate the deal, Iran scales back its own commitments. All of this is in the existing treaty.
And there is no guarantee that meeting the new demands would result in sanctions stopping, rather than in more sanctions and demands.
