About War Driving ..

Hi ,
I was wondering if it is possible to locate and catch
a guy who is connecting to our wep wireless network
and downloading stuff from torrents and using up our
bandwidth ..
I checked up with arp scan and found 2 unknown IPs and 247
Is there anyway of locating the guy in a building of 7
floors and how to stop this ..I have tried changing
the Wep keys so . he is cracking the wep key.
Any Suggestion People ?

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