There is no option to turn BTWifi/ Openzone 'off' as such as it is a feature on your package that allows you to connect to it when you are away from your premises. However, it is dependent on people broadcasting the service from their router - on a BT router it is possible to disable the service at your premises - which would prevent other people from using it if they are in your area. Before anyone worries, it only uses a tiny proportion of your broadband service so has no noticeable impact on speed, doesn't have any impact on your broadband usage allowance and is perfectly secure!
