Shove will break it, you can fireball it, I also used to be able to knightly leap onto the same square and then just click it. I'm sure there are other ways.
You could kick an arrow trap over to be "underneath" the statue (i.e. the statue is between the trap and the gargoyle) then activate the trap. What's weird is that you can't shoot these statues with normal bolts; you have to use some kind of spell effect or teleport on top of them.
Yeah, that's always bothered me. Similarly, you can use Unliving Wall or Psychokinetic shove on a square (to say, break a statue or push an artifact), but the respective wands don't work the same way.
You haven't noticed? Those statues disintegrate even if you only poke them, they must be made of ash or something.
Hell just a Dragon's Breath will break one.. the only way you CAN'T break it is with a bolt or regular throwing weapon.