How can I stop a nearby property from accessing my wi=fi and using up my data?please help.
Ensure your Wi-Fi is using WPA2 (or better) encryption and change your password.
Use a web browser to log into router's admin page and change the WiFi password. Pick something long with numbers and symbols.
You will have to reset the password in all your devices.
Don't write/post the password visibly on any noticeboards.