I've tested all these attacks myself, and with a good directional antenna with a high transmit power the attacker can be pretty damn far away from you.
Even if you lower your router's power output (a very good first step to mitigate this attack), his directional antenna will allow him to pick up fainter signals.
Disable 2.4GHz if you can, and just use 5Ghz as there are far fewer high powered directional antenna available. The 5GHz signal also doesn't propagate as far.
If you find the location he's coming from, you can shield that with foil.