Windows 8 by default automatically connects to the previously connected/known Wi-Fi network. Listed below are steps to disable the automatic connection to known Wi-Fi network.
Step 1: Right click on Wi-Fi icon and select Open Network and Sharing Center menu option.
Step 2: In the Network and Sharing Center window, click the Wi-Fi connection link.
Step 3: This should display the Wi-Fi Status windows as shown below.
Step 4: Now click the Wireless Properties button in W-Fi Status.
Step 5: Un mark the check box with label as Connect automatically when this network in range option available under the Wireless Network Properties window. Click Ok button to confirm and the save the changes.