For a given Location, I only want to connect to one specific WiFi connection. How?
Source:
Here's how to forget a wifi location so that you don't auto-connect to it when in range.  


Click the Wi-Fi icon toward the upper right of your screen in the menu bar. It looks like a pizza slice.
Click on Open Network Preferences.

Click the Advanced button.
Click the Wi-Fi tab.
Select the network(s) you'd like your Mac to forget.
Click on the minus (-) button.
Click on the OK button.
Click on the Apply button.