You sure its not just someone or something that your giving the password to? Like a BR player or a phone? Or maybe a friend of a family member that lives near by the someone is giving the password to? Its not kids stuff to break WPA2. Also it would likly show up in your router logs too, unless they are just very good at guessing. Yes, it can be done other ways but brute force, I think thats beyond the realm of probability here. But if you want to increase your security, here are a few tips.
1. First, go wired for a few days. Stop wireless all together. Chances are, with no target, they will get bored and move on.
2. While wired, set "Enable Graphical Authentication" and choose a long random admin password.
This is a good site to use to pick one. Just be sure to save your router's configuration and write the password on a sticky you stick to your router.
3. When you enable your wireless again, set your SSID to Invisable. For anyone that can break WPA2 this doesn't slow them down, it just makes you less of a tempting target.
4. Set your security to WPA2 with AES only cipher(assuming your network is compatible). Not only is this more secure but can give you tiny tweak in performance.
5.Turn on Mac Filtering and Only allow those in your list to connect. This will make connecting your new devices you may get harder to connect, but give a decent speed bump to anyone trying to access your network.
6. Decrease your DHCP IP range to just the amount you need. Be sure to set DHCP reservations though to stop possible IP conflicts.
7. Decrease your DHCP lease time and turn off DHCP broadcast. This is over kill I think, but it does make it a bit harder for them.
8. Decrease your Group Key Update Interval. Its in seconds. Anything under 400 is probably probably little gain, and this will cause possible problems with your own network, particularly devices that go into standby or use power saving modes (IE Iphone).
You could go even further two with 3rd party firmware, exploring WPA Enterprise or building your own firewall, but the effort to gain ratio is not really in your favor. And as you see, the suggestions get harrier and more troublesome as you go down that list. Anything past 5 may give your some headaches, so be warned.
PS. Another option is to monitor instead restrict. Make your network wide open then use a packet sniffer and set up some parental controls to monitor sites. He may just be some Copy pasta script monkey that once connected, doesn't know anything else. If so, it may be easier to just find out who he is and knock on his door.