macOS automatically rejoins any Wi-Fi network you've previously connected to when it's in range. If you no longer want your Mac to rejoin a specific network — like a former Downtown Works network, a neighboring open network, or a phone hotspot — you can make it "forget" that network.
Steps (Current macOS)
Click the Apple menu, then System Settings.
Select Wi-Fi from the sidebar.
Click Known Networks (or scroll to the network list).
Find the network you want to remove, click the (i) info icon next to it.
Click Forget This Network, then confirm.
On Older macOS Versions
If your Mac shows System Preferences instead of System Settings (macOS Monterey or earlier), go to System Preferences > Network > Wi-Fi > Advanced, select the network under Preferred Networks, and click the minus (–) button to remove it.
After Forgetting a Network
Your Mac will no longer auto-join that network, even if it's the only one in range. You can always rejoin manually later by selecting it from the Wi-Fi menu bar icon and re-entering the password if required.