Is there an option I'm not seeing to turn off spellcasting in autocombat?

I seem to remember this also being a problem in previous games in the series.
In this one where I'm hoarding mana to try and push back the doomsday clock, it's ridiculous.  
I just went from about 100 mana to -70 in a fight.  Yes, minus... AI felt just wasting it all on one easy-ish fight wasn't enough.

So basically I can never use autocombat, and it makes the game tedious.  

Please someone tell me I'm missing the way to fix this, if not I hope they patch it (but I hoped that in the last games, too).