This article will explain how to disconnect, or forget, a saved WiFi network from a macOS device. (iMac, MacBook, etc.)
1. Begin by clicking the Network symbol at the top right of the screen and select Open Network Preferences.
2. Select Advanced... in the bottom of the Network window that pops up.
3. Click on the WiFi network you want to forget, and click the minus (-) button to forget it.
4. You can verify that the network forgotten by making sure it doesn't appear in your list of preferred networks. Click OK to continue.
5. Click Apply on the bottom right of the Network window to save your changes.