Erdogan has decided that Russia is at war (very perceptive of him).  Therefor he has the right to prevent Russian warships (and Ukrainian ones too) from passing the Dardanelles.

Under the 1936 accord, Turkey has control over the Bosphorus and Dardanelles and the power to regulate transit of naval warships. It also guarantees the free passage of civilian vessels in peacetime and restricts the passage of ships not belonging to Black Sea countries.

In wartime, or when it is threatened by aggression, Turkey is authorised to close the straits to all foreign warships. It can also refuse transit for merchant ships from countries at war with Turkey and to fortify the straits in case of conflict.