The ONLY way to stop the thing connecting to your FON... SSID from time to time is to delete the relevant profile.
To get windows to forget for example the BT...FON Wifi SSID
In a run as "Administrator" Command Window type
netsh wlan show profiles
C: \Users\T>netsh wlan show profiles
Profiles on interface Wireless Network Connection 2:
Group policy profiles (read only)
---------------------------------
<None>
User profiles
-------------
All User Profile : BTWifi-with-FON
All User Profile : AndroidAP99
C: \Users\T>
Make a note of the exact name and capitalisation of the one you want to delete. Then type eg.
netsh wlan delete profile name="BTWifi-with-FON"
This will stop the device from connecting to that WiFi name automatically unless it is once again manually selected from the list of WiFis AND the "Connect Automatically" box is checked.
By the way - If you delete a profile that has a WiFi Key associated with it the WiFi Key will need to be re-entered if you wish to connect again.
You can otherwise safely delete any and all of the profiles.
I would delete any that you do not anticipate using again.
I suspect that -
netsh wlan delete profile name="*" will work.
This will though forever forget all your saved WiFi Keys.
Good luck.