Usually there is a button which switches the compressor on or off so that you can just have 'fresh' air; in the case of automatic A/C you can usually switch off the Auto function which again means its just ambient air...
An actual conversation I had with my Miata, with automatic climate control:
Me: "I would like outside air, please"
Mi: "Certainly, sir, simply switch from 'recirculate' to 'fresh' on the button"
Me: "Done. But the air is still being heated by the heater core it seems"
Mi: "My apologies, sir. You may reduce the temperature as you see fit"
Me: "Done. But now the AC compressor has switched on"
Mi: "Quite rightly, as you asked for colder air. If you would like warmer air, simply..."
Me: "No, no...I don't want warmer air, or colder air. I don't want you to do anything to the air other than blow it on me."
Mi: "Oh, I believe I see. You want outside air blowing, unadulterated by my climate control?"
Me: "Yes, that's it!"
Mi: "Then might I humbly suggest sir PUT THE BLOODY TOP DOWN ON THE CONVERTIBLE HE BOUGHT"
1) My car's use of "bloody" indicates this little Japanese roadster has a British soul.
2) I would still like to be able to pass through outside air, unheated/uncooled, without having to guess what temperature setting achieves that. It's not 100% clear that's actually what happening.