How do you destroy a floating cube?

[Middle Twin]

I had a floating cube spawn in a crater right next to my biggest army, so I walked over and there was a fisherman and a single horror. I killed them, but nothing happened so I walked away. A few turns later I passed by with an army with a scout and there were a ton of evil trees, clearly spawning faster than I would be able to hire troops to kill them.

I reloaded an earlier save from before the cube which I thankfully had lying around, but it got me wondering: Is there any way to prevent the mass tree spawn once you have control of the cube tile or are you just screwed?

[Gilmoy]

During the game, type '?' to show a list of all hotkey commands.

The one you want is 'o' for Use location's special power.
Every destroyable special location enables that command to destroy it.
Stand in the floating cube's crater square, and use that.

This command also appears in the context-sensitive window of commands at upper right,
when you're in a square that enables it.