How to block certain IP/MAC Address that access your WiFi Network ?
On your airport extreme you can block certain mac address for good with "No Access".

In your airports configuration go to the "Network" tab and check "Enable Access Control", then click on "Timed Access Control".



Then on the top add the mac address you want to prohibit and give it a description.  Towards the bottom, On the "Wireless Access Time" drop down select "No Access" and save.



Confirmed this works on a 5th generation airport running version 7.6.4