How do I cancel a shot?

I order a shot on the enemy weapons, but then their shield goes back up and suddenly I'd rather wait until the rocket can get through to take it out. Or maybe their cloaking device suddenly comes on. Anyway, I want to cancel the shot right away. How do I do this?

I tried right clicking on the target, but the only solution I can see is to turn the weapon off and on again (wasting the charge).

First Pause the game using spacebar. Then click the weapon icon you intend to cancel the shot from as if to target it to another system, then right click somewhere in empty space to cancel the shot entirely.

For the record, you can turn the weapon off/on without losing the charged weapon state if the game is in paused mode, so this is another way to cancel shots.