Mercury can be present in different forms, unfortunately these are confused and mistaken by most media outlets and most people commenting on the situation. Metallic form (liquid metal) is rather easy to remove, as it has a very high density and is always on the bottom, so you can separate it my just be mechanical means (technically it is called decantation - dekantacja in Polish). Then there are vapors - metallic mercury is highly volatile and the vapor will be always present whenever there is a metallic mercury. But broadly speaking the toxic form is oxidized mercury, in a cationic form - it is water soluble and many of its compounds are volatile. There are techniques that can be used to remove heavy metals from water, but I doubt they will leave whiskey palatable. Note that whenever there is the metallic/gaseous form present it gets slowly oxidized and produces toxic compounds, so even if removing metallic form is trivial it doesn't mean whatever was in contact with it is safe.
From my understanding at the moment mercury in Odra waters is like a Schroedinger cat - it is both reported as present, and reported as not present. Hardly funny, but there is so much speculation about what really happened, and so much contradicting information, that I am still waiting for some credible data. The only thing known for sure is that the fish are dying.