Former US Assistant Secretary of State Victoria Nuland was denied a Russian visa, because she is one of the organisers of the Euromaidan in 2014.
The Euromaidan protests were a spontaneous reaction among large parts of the Ukrainian population, ignited by the decision of former President Yanukovych not to sign the Association Agreement with the European Union in November 2013. More similar cases here.
Former US Assistant Secretary of State Victoria Nuland was indeed denied a Russian visa. Sources in the Russian Foreign Ministry reported that Nuland is on the "black list", drawn up in response to US sanctions against Russian citizens. Nuland applied for a Russian visa to take part in a closed international conference in Moscow from May 20 to 21.