Not real familiar with it, but can’t the Logan Act be used here?
Something certainly should be. According to Wikipedia, the Logan Act "is a United States federal law that forbids unauthorized citizens from negotiating with foreign governments. It was passed in 1799 and last amended in 1994. Violation of the Logan Act is a felony, punishable under federal law with imprisonment of up to three years. The text of the Act is broad and is addressed at any attempt of a US citizen to conduct foreign relations without authority. However, there is no record of any convictions or even prosecutions under the Logan Act." [emphasis mine]