Step 1: Isolate. Use a spare PC, add a NIC and use Untangle Lite (free) http://untangle.com/ which has very good. Turn off DHCP in your router, use it as an access point only. Let Untangle hand out addresses. Get the perp's MAC address and reserve his IP addresses. Use Untangle's report feature to build up a dossier of all his activities over a few weeks. See what he's doing.
Step 2: While compiling the reports, use HeatMapper (free) http://www.ekahau.com/products/heatmapper/overview.html on a notebook or netbook to locate him. It won't be any problem to find his AP in the signal map.
Step 3: After you have the data, mail him a copy of the reports and the heatmap to let him know you know what he's doing, and invite him over for a cup of coffee or other beverage of your choice. Be sure to tell him you don't want to turn him in or blackmail him, but you would like to talk geek to geek. Tell him you're going to disable WPS and change the WPA key, but you'd like him to try to hack in again, and tell you if you've left any open vulnerabilities. You can end the leeching and might just gain a buddy worth having.
Caveat: Of course you want to send a copy of the report to someone else to hand over to Law Enforcement in case he turns out to be a terrorist or freakazoid with implements of destruction to use against you.
Chemist who falls in acid will be tripping for weeks.