Stop wifi connecting to open networks
It shouldn't be connecting to WiFi networks unless you have previously connected to them.

If you go into Settings and view your WiFi networks you should see a list similar to:



The ones that are listed as 'Not in range' are networks you have connected to previously. If these are the open networks that you no longer want to connect to you can click on each one and click the 'Forget' button that appears.

Your phone should not connect to these networks again (unless you tell it to).

EDIT: After your edit it appears like this question will help.