How to permanently stop Samsung Galaxy S5 from keep connecting to Wi-Fi?

I'm using Samsung Galaxy S5. At my place of employment, Wi-Fi is provided (a free version which isn't locked, and a version which is locked). It keeps connecting to the unlocked Wi-Fi network. I don't want to connect to either. I want to control which networks I connect to and when, whether they are free or not (especially if they are free as I imagine that's how hackers get access to your personal info). I went into Settings -> Connections -> Advanced and turned off almost everything I could think of, but it still scans for networks and connects to the open free network. How to stop the phone from automatically connecting to a Wi-Fi network and instead only connect to a Wi-Fi network when prompted by me?