In 1812, when Tsar Alexander broke the continental blockade, could Napoleon just have installed some powerful canons in the strait of Denmark to prevent Petersburg to receive ships from UK? I assume that trade throught Arkhangelsk is minimal due to the distance to inner Russia, same as for Caucasus/Iran/Central Asia. The other option being Crimea but afaik, the Ottoman just were at war against Russia and may not agree and certainly sees the French as a powerful threat.