How to block certain IP/MAC Address that access your WiFi Network ?

I can see people access into my Wifi by going into AirportUtility Here is a list of people that are connected to Wifi at the moment.

There're 2 of them in there that I'm not sure - how they get in there ?

Goals

• I would like to both of them out.


• I also want to block a certain MAC Address for future access.

What I have tried

I've been playing around with the AirPort Utility, and all I can do is restrict the time access to a certain MAC address.

I also came across an app called Little Snitch, but that not exactly what I want.

Is there any Mac App that let me block/allow a certain IP address ?