How on earth do you exit Quick combat mode once it is activated? The manual just says that you interrupt it, but how do you do that? I have tried getting in other commands like move while in combat but that does not seem to do it.

Try this:

"You may take manual control of all characters by typing the spacebar while one of your characters is moving on
the screen."

So, turning off quick combat is actually one of the features of Gold Box Companion, among others. It's actually the reason I first downloaded it.

http://personal.inet.fi/koti/jhirvonen/gbc/

GBC does a lot of other stuff too, like allow full character editing. So if this is too tempting to you, maybe try another solution.

Are you sure it's Gold Box Companion to stop Quick mode?

I couldn't bear DOS UI so to replay I picked Mac PoR (the version I played in past) which has a full mouse support. It can be played only with mouse, return... and spacebar to stop Quick mode.

You can just keep the spacebar pressed because try synchronize character playing and press the spacebar can be a bit tricky. It's really handy to be able use quick for some combats.

I believe ou can turn off quick combat in vanilla but it always was unintuitive or not too evident or maybe too fast because of the DOSBOX cycles.
What GoldBox Companion does is Autodisable quickfight after combat. Some of the older games didn't disable quickfight between fights so you could enter a dangerous fight with quickfight on.

This is the right answer. Ignore the other answers.