Use the strongest available wireless protection, and you dont actually need to buy any software, if ya use your mind, simply you'l know how far he is because the strength of the signal.
#2. i cant explain it, since backtrack does not allow ilegal (if it is, here it isnt) activitys
#3. umm google maps or some ip tracking thingy blah blah blah, for that you got 3 more ideas i could get easily to find someone down, but its not always correct and haves errors in most cases, so, to be a good advisor, simply change the strength of your signal :P
btw, we got a cop in town
(edit#2) He believes.
Go down to your Router's (Modem's) configuration page and see the DHCP Client List & also check the MACs that had been connected to your Router (Modem).
Have fun !