Forget a Wireless Network on an OSX Device

Click the wireless icon in the upper right corner and click Open Network Preferences... at the bottom of that menu

Make sure Wi-Fi is selected on the left and click the Advanced... button

Under Preferred Networks, click the network you want to forget.

Click the minus (-) button below the box to remove the network then click the OK button.

Click Okay and Apply in the previous Network window to save the changes. You may also need to turn the Wi-Fi off and back on again to see the change.