In your case, you can login to the router's configuration page and do the wireless MAC filtering settings which is under wireless tab.
Just add the client that you decide to block into the blacklist. What you need is the client's MAC address.

Like you, I've my neighbor in good relationship that I don't want to loose. I have a limited internet connection he wants to use via my Wifi device so I send my password to him but I added all of his devices MAC addresses in blacklist. Furthermore, I dropped my wireless antenna downward so wireless signals always blink to him.. lol.. :D