Try this:
Remove the device from here then reset the password via here and select the second option, "reset via security question" . After you've set up a new password, you should be asked if you wanted to review your linked apps. Click on that and unlink all apps linked to your sky go. Once you've unlinked them, everyone who had access before will be asked to sign in when attempting to use your sky go. If this doesn't work try resetting the password via Sky Go where you login to it, select the forgotten password option.