I started quick mode combat during an obvious action point, and want to stop my characters from using this autobattle now. How do I stop the quick combat command from executing?

Press Space.

This was answered above, but I'll expand. You need to "interrupt" your character before the CPU performs the action. You can do this by spamming the spacebar before their turn.

So, after performing an action on a controlled character (eg. Done > Guard), hold down the spacebar immediately (while the text still says "GUARDING") and you should resume control of the next character if you done already.