What use would "cannot be placed" even have?
My favorites out of these are the last two, those are both clever
Surely the opponent would be smart enough to never steal such a stone... you'd need to expand the effect to be something like "whenever your opponent alters your hand, this stone is moved to their hand instead", so if your opponent uses something like 37 or 61 or 89 then instead this stone gets moved to their hand.