
Click on the Manage known network option, and for each wireless network listed that you no longer want to use, click on it and then click Forget. In the case that you have connected to several access points before, and have the correct passwords, but you don't want to use them now, you can use the Settings, Network & Internet, Wi-Fi page to remove access points you no longer want to use. I will use Windows 10 as a working example. You do not mention what OS you are using.

Are you dealing with an environment where your PC is capable of connecting to multiple access points, but you only want a connection to a specific access point, in your example only A? Or do you mean that even though you are already connected to access point A, you do not want your PC to list other available access points that you could connect to but that you do not want to use? If you mean something entirely different, please add more information so we can understand what you are trying to achieve. Your question is not exactly clear on what you are attempting to accomplish.
