An important step in troubleshooting wireless network issues is to isolate and remove any potential conflicts. These steps will show you how to forget wireless networks you don't use for most versions of Mac OS X:*
- Open System Preferences
- Click Network, then Advanced…
3. Select a network you’d like to forget and click the “-” icon as shown below. You can select multiple networks by holding the command key while clicking and batch the process.
And that's it!
*This article features images and content from https://9to5mac.com/2018/07/20/mac-how-to-forget-wireless-networks/