Welcome to the Forums Biblofilter!
Its always fun to push creatures into walls! As far as I know, its not possible to push flying units into the arena floor although thematically that would make some sense. Flying units ignore walls in general. The only thing you can push flying creatures into is the arena walls. For everyone else, you can push creatures through and against certain walls, as well as pushing them into the Arena wall and inflicting that 3 dice extra attack.
If you push a creature into a wall with the passage blocked trait, they just receive the 3 dice attack. If you were using say [mwcard=MW1I12]Force Push[/mwcard], and you wanted to push a creature through a wall with a passage attack, according the spell, you need to pay an extra 3 mana to do so. That creature would then be pushed through the to the other side of the wall and receive the attack stated on the Passage Attack wall. Not all pushes require you to pay mana to push a creature through a passage attack wall. Take [mwcard=FWC13]Selesius, the East Wind[/mwcard] for example. Her second attack has the chance to push creatures, but doesnt require you to pay any mana if you so choose to push a creature through a passage attack wall.
Passage Attack walls:
[mwcard=MW1W03]Wall of Fire[/mwcard] -
[mwcard=FWW01]Wall of Pikes[/mwcard] -
[mwcard=MW1W04]Wall of Thorns[/mwcard] -
[mwcard=DNW01]Bloodspine Wall[/mwcard] -
Passage Blocked Walls:
[mwcard=DNW02]Wall of Bones[/mwcard] -
[mwcard=MWSTX2FFW01]Wall of Earth[/mwcard] -
[mwcard=MWSTX1CKW01]Wall of Steel[/mwcard] -
[mwcard=MW1W02]Wall of Stone[/mwcard] -
Arena Walls
Hope this made some sense.