[iMore.com]

By default, your Mac will remember settings for any Wi-Fi networks it's connected to in the past. This can make things really convenient as you move around; if it's for networks that you no longer have access to or don't want to use, however, it can be really inconvenient — because sometimes your Mac will try to connect to networks it can no longer work with. Here's how to fix it.

Full story from the iMore Blog...