How do I disable a discrete Nvidia graphics card?

BIOS

Since the question is about disabling the discrete video card and it was mentioned that there is no such setting in the BIOS, I answer it. In your BIOS, and as far as I remember, in some of the comments under the questions you wrote that you have it AMI BIOS (I shorten it to an abbreviation), you need to do the following steps.

When you go to the BIOS, you will have a blue-gray menu, the following items should be in the upper navigation bar:

Main | Advanced | Power | Boot | Tools | Exit

The menu itself is at the top, in the AMI of different years may be different.
Next, go to:

Advanced -> Chipset -> SouthBridge Configuration

Next, click on Primary Graphics Adapter, a window should pop up with map selection options. IGP - respectively responsible for the integrated graphics card, and PCIE for Nvidia in your case. Choose the one you want to activate. There with which it starts, it will be respectively in priority, after the selection.

Next, you need to go to the general menu level and select

Exit -> Exit & Save Changes

Well, then just restart the laptop with a new configuration. It seems to be nothing complicated.

Required system data to fix the problem:

As for the Debian 9 OS itself, you can output data about the BIOS itself in the terminal by using the command (showing everything from root):

# dmidecode --type 0  

Data about the motherboard:

# dmidecode --type baseboard  

To gather even more information, you can use lspci - this is a standard tool for querying devices connected to any PCI-compatible bus:

# lspci -nn    

You can also reduce the output to:

# lspci -k | grep -A 2 -i "VGA"  

If nvidia-detect is not installed, you can install it and collect data (although data from lspci should be enough) by typing in the terminal:

# nvidia-detect

Using the above commands, you can collect all the necessary information for further work on fixing the problem.

Nvidia

If you need to completely remove the drivers from Nvidia, it is quite simple to do. Run the following command to delete all packages that start with nvidia in the package name:

# apt purge nvidia-*