Whenever your Mac connects to a wireless network, its SSID and password are remembered. As your Mac comes within range of that network, it automatically connects. In order to prevent your Mac from automatically connecting to a network, it can be removed from your saved network list. If you are trying to connect to a password protected network, and it shares a name with a previously used network, it may have to be removed as well.
To access your saved Networks, open System Preferences from the Dock or Applications folder.
Under the Internet & Wireless section, click the Network icon.
In the Network Preferences window you will see the wireless network you’re currently connected to, a checkbox for Asking to Join Available Networks, the Show Airport Status in menu bar, and the Advanced button. Click Advanced.
Here you will see a list of your “Preferred Networks,” or networks that you have connected to in the past. Scroll down to a network name that you want to forget.
Highlight the network by clicking on it and then press the Minus - button to remove it from the list. You can also rearrange the priority of networks here. Dragging a network higher on the list will make your Mac connect to it before any others in the area. For example, if both Linksys and Netgear are in range of your Mac, and Linksys is higher on the list, your computer will connect to it automatically.
