Saeviomagy
Adventurer
I don't see what the problem is: the rules say that ships and other objects are not enemies, allies or creatures. Powers and effects only work on enemies, allies or creatures.
Hence, if you say "I punch the ship to death", the DM says "your power doesn't work on the ship".
Admittedly it gets a little silly when people want to burn a ship with flaming sphere and the DM says no. But the DMG advises that he should probably say yes if the specific case warrants it.
There's almost no danger of your players punching the dungeon down unless the DM thinks it's cool and allows it. And even then, there's no guarantee it will work a second time.
Hence, if you say "I punch the ship to death", the DM says "your power doesn't work on the ship".
Admittedly it gets a little silly when people want to burn a ship with flaming sphere and the DM says no. But the DMG advises that he should probably say yes if the specific case warrants it.
There's almost no danger of your players punching the dungeon down unless the DM thinks it's cool and allows it. And even then, there's no guarantee it will work a second time.