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There was an SSH thing I picked up somewhere years back, probably one of the "Dumb Terminal Tricks" from http://www.osxfaq.com/, where you could type a message that would pop up in the a little window in front of everything on the target Windows computer. I think this required knowing the username/password of the target, but if you could get ahold of that, what fun!
If they're running OS X, there's a simple way to toss up a "greeting" every time they log in to a machine, though I don't know if this can be done when they simply access the network. When anyone logs into my machine at work, I give 'em a nice "Good morning, Dave! Nice to see you again."
When the mister discovered our previous neighbour's wireless was wide open, he was kind enough to let them know and told them how to lock it down. I believe a lot of the stealing of wireless signals is out of ignorance like the neighbours, but there are real thieves out there too.
You can hardly go anywhere urban these day without finding an unlocked network named NETGEAR on the list. People take the things out of the box, plug them into the cable modem, and done!
I lock mine down because I worry that someone will d/l illegal stuff on my IP address, then I'm the one raided by the FBI or the RIAA jackboots.
I cannot tell you the sheer number of times that cartoon got forwarded to me by friends that assumed I was Mrs. Roberts because we're forever doing funny things to people who use the open network for very long or who try to get around the DMZ rules. (We leave one open because our property backs up to a neighborhood park where everyone brings their kids to fish and play with the ducks. I don't have any issues with folks using it to check email or read the web or whatever, but I have some fairly hard rules about downloads and trying to get around my blacklists.
I want, more than almost any other web art I've seen, a signed copy of that cartoon on my wall.