http://www.bbc.co.uk/news/world-europe-44153923
Poland's internal security agency, the ABW, said " C" would be expelled soon. Four other people are now barred from entering Poland.
ABW spokesman Stanislaw Zaryn said the agency had "neutralised" two "Russian hybrid war networks" targeting Poland.
"Hybrid" war includes spreading fake news to change public attitudes.
Poland has been on high alert for Kremlin attempts to influence Polish opinion since Russia annexed Ukraine's Crimea region in March 2014.
Poland - a member of the EU and Nato - is one of the strongest critics of Russian foreign policy, especially Russia's military intervention in Ukraine.