Uh? Sounds like it's your network that is insecure rather than theirs.
If your WLAN is set up correctly then noone will be using it (unless they're determined hackers which is very unlikely)
A poorly confirgured WLAN on his side will not be giving him access to yours.
If his is insecure, why worry about it? It's their own responsibility.
---------- Post added at 15:07 ---------- Previous post was at 15:01 ----------
1.) You should only be allowing defined MAC addresses.
2.) You should turn "broadcast" off.
3.) Make sure you impliment encryption of whatever type. (WPA preferably)
For a
home wireless setup that should be enough,
very few people out there will ever get someone trying to hack in and steal bandwidth.
There is absolutely no need to contact this person.
If your setup is being used, then secure it. Chop them off, simple as that.