More to the point, it doesn't actually qualify as copying. If you read the GPL, it talks about making copies, which the car's manufacturer did. Joe is transferring the copy he received from the manufacturer. He's never copied the software.

Simple:

Switch to your preferred Unix-like OS (BSD or Solaris is probably better than Linux for this--most people have heard of Linux) but get rid KDE, GNOME or any other friendly desktop environment. Instead, run your preferred window manager (something that looks nothing like Windows or MacOS is better) and use xterms for all your actual interactions. Use lynx as your web browser and mutt as your mail client. (Or if you must use Firefox/Thunderbird, find the most outlandish and confusing theme and set of extensions you can find.)

Then, when somebody asks to use your laptop, just say "Sure, go ahead. This runs $FREE_OS, though, which may not be what you're used to." If they ask for help, tell them which man page they need to read to get the answer.

After three or four iterations of this, nobody will bother you anymore.

(Also, it's a good idea to set up a guest account and some way to to it, just in case you meet somebody who knows Unix. See other posts in this thread for more suggestions.)

I just downloaded it and played with it for a few minutes. Some comments:

1. I don't think your license actually meets the criteria for open-source. It isn't clear to me that I legally could create a fork of Sandbox and distribute it under the same terms. I'd be much happier if you used a well-known open-source license (e.g. the GPL) instead.
2. I also don't how your license claims the copyright of any submitted changes. A more suspicious mind might think you're trying to build a base of contributor games that you can then commercialize. If that isn't your plan, I suggest you change the terms to make that clear. Once again, using a well-known OSS license would solve the problem.
3. Aside from that, I think this is a really neat idea for teaching kids to think like programmers.