From Reuters via The Telegraph:
"[Russian Foreign Minister Dmitry Rogozin] suggested Russia could use the station without the United States, saying: 'The Russian segment can exist independently from the American one. The U.S. one cannot.'"
http://www.telegraph.co.uk/news/worldnews/europe/russia/10828964/Russia-to-ban-US-from-using-Space-Station-over-Ukraine-sanctions.html