An update for anyone that was interested/has the same issue.
I soaked the hydro stone in water for some time (a day or so) but although the water seeped out a lot of aroma the stone itself didn't really improve much
I then boiled the stone on the stove in an open pan of water for an hour. Improvement definitely noted but still heavily ghosted.
I soaked the stone in white vinegar. I left it on a shelf for a few days (3 or 4 I think), cleaned it with fresh water and an old toothbrush, then boiled it for 20 minutes or so until the vinegar smell had gone. Allow to fully air-dry. The stone is now aroma free (near as damn-it)
My advice: If you use hydro-stones or terracotta pot shards. have several and mark them for different types of tobaccos (mild, medium, smokey, aromatics etc.) or else do what
@simong does (check reply above).
Happy hydrating!