I can think of a few much easier ways to prevent unauthorized wifi usage that don't run into the potential for getting charged with computer crimes yourself. I don't know how the law would see you collecting passwords, and I don't see it as worth the legal risk.
First off, if you ran WPA2 encryption, they would not be able to log on without either knowing or cracking your key/passphrase, and if someone unauthorized DOE manage to figure it out, you can just change it, give the new login credentials to authorized users, and lock the unauthorized ones out again. (I would steer clear of WEP as that is ridiculously easy to crack.)
Then there is a little thing called "MAC filtering". Even if somebody cracked my WPA2 key, they would not be able to access my router via wifi, or even if they snuck in to my living room and plugged in a CAT5 cable. There are only three computers that can. When one of the computers was replaced, we had to reconfigure the router to allow it on our LAN.
And there is a reason I don't broadcast my router's SSID either. Most people do any *ANY* wifi computer can at least know the name of your router and what encryption is uses. If you don't know the name of my router, you aren't going to find it very easily. It won't show up on the list of networks within range.
All of these things are in the manual for your router. Despite all of the advances in computer networks and the Internet, sometimes the most important information is printed on a flat sheet of dead tree.
