You need to check what encryption you are using. None allows anyone to log in. WEP is flawed and can be hacked into in minutes. The various flavours of WPK are pretty secure and hard to hack. Log in to your home hub (http://bthomehub.home/) and look around for the Wireless settings. You can change the encryption and wireless key as desired.
Pick a really obscure wireless key for best security. You will need to disconnect and reconnect any wireless computers.