How do i delete devices from my WiFi and make sure no one else connects?
The xFi app has the option to delete a device.
Do not give out the SSID and password for your wifi to anyone who you don't want to use it.
Years ago many routers had MAC address filtering (blocking/allowed) but ultimately it does not accomplish much to someone who really wants on your network and has your WiFi password already. Basically MAC addresses can be faked so the filters don't work.
There really is no other system to keep someone off your WiFi. If they already have your network login information your only choice is to change that password to something new.