Just curious if it's Possible.. In our Quest to find the source of the Biggest User of GB's in the Household... Can and if so How could say a neighbor Hack into our WiFi Signal..?

Wireless is inherently insecure and relatively easy to utilise.

Depending on your router there may well be more granular security and data controls available.
As well as a Wireless key (WPA) the use of MAC filters can be useful.