The administration can't stop her. There are 3 equal branches of gov't. The exec. branch can't interfere with the Legislative branch.
The President has exclusive power to negotiate with foreign countries, per Article II, Section 2, Clause 2 of the Constitution. He has exclusive power to appoint ambassadors, permanent or otherwise. He has control over visas. He thus has power to approve or deny congressional visits to a hostile nuclear power, especially in time of war. You will find no enumerated powers in the Constitution authorizing members of Congress to negotiate with a foreign government.