0) Any network including wireless network (aka WLAN, WiFi)

1) Not thru iTunes Sharing

2) Mac user go here and Windows user go here

3) You can't (see also #2)

4) Huh? LimeWire shares only files that are in the Shared folder or rather what you've configured in the Settings. And it makes sure nobody can access files that are outside of this folders thru LimeWire.

There are however so many ways to gain access to a computer/network that are not related to LimeWire. Bad passwords, security holes, buffer overflows and the #1 reason are not properly configured network applications and devices. The latter seems to be your case! John is either connected to your Wireless Access Point or you are maybe connected to Johns. Wireless Network Cards with default configuration tend to pick Access Points with the strongest signal.