The Stubborn Connection
Ubuntu uses NetworkManager to seamlessly handle your internet connections. When you successfully connect to a new Wi-Fi network, NetworkManager saves the password and automatically flags that network as a trusted, auto-connect profile. This means that whenever your Ubuntu machine detects that specific Wi-Fi signal in the future, it will instantly try to connect to it. While great for your home router, this is highly problematic if you connected to a slow, unsecured public cafe hotspot once, and now your laptop stubbornly drops your fast mobile tethering connection to auto-join the terrible cafe network every time you walk past it.
Disabling the Auto-Connect Flag
To retain control over your active connections and stop Ubuntu from automatically reconnecting to specific, unwanted Wi-Fi networks, you must edit the individual profile settings for that network.
Click on the network icon (Wi-Fi symbol) in the top-right corner of the Ubuntu top bar to open the quick settings menu. Click on the small arrow next to your active Wi-Fi connection, and then select Wi-Fi Settings. In the Settings window, look under the “Known Networks” list. Find the specific network you want to stop auto-joining and click the small gear icon next to it to open its properties. In the new window, ensure you are on the Details tab. Look for the checkbox labelled Connect automatically. Click this checkbox to remove the checkmark. Click the Apply button in the top right. Ubuntu will remember the password if you ever want to join it manually, but it will never connect on its own again.