Mac users can quickly disconnect from a wi-fi network by using the wireless menu in OS X. This simple task is incredibly useful for managing and juggling multiple networks, whether for something as simple as using iPhone Wi-Fi Hotspot or even a more advanced task like packet sniffing.
How to Disconnect From a Wi-Fi Network in OS X Without Turning Off Wi-FI
This is only disconnecting from an active wireless network, the wi-fi function will still be enabled and usable for other purposes:
1. From anywhere in OS X, hold down the OPTION key and click on the Wi-Fi menu bar item
2. Identify the currently connected wi-fi network by name, it will have a little checkmark next to the SSID of the router
3. Directly underneath the wireless routers name you will see the newly revealed “Disconnect from NetworkName” option, simply select that to disassociate from the active wifi network
The network disconnection is immediate, and as usual when detaching from a network you will lose internet functionality and access to network assets. But, critically, your wi-fi card is still enabled and has not been turned off, meaning you can still interact with wi-fi networks. If for whatever reason you want to turn off the actual wireless hardware, just select ‘Turn Wi-Fi Off’ in the same menu item, there is no Option key modifier required for that.
Now that the Mac wireless card is disassociated from a network, you’re free to do whatever task requires the wi-fi card free from a network.
Source: osxdaily.com