[X4U] Wireless "theft"

[Gene Wieber]

I just got a note via my printer "THANKS FOR THE FREE WIRELESS!" I have a
Mac connected to a Linksys wireless router linked to my DSL modem and Mac
connected via Airport Extreme. In both Macs I had printer and file sharing
on. So, my question is is my entire system open to whoever is within my
wireless range? Would having the printer and file sharing off prevent
further intrusion?  What should I do for better security? I'm really new to
the wireless world, as you can tell by the questions. Thanks, Gene