On the Mac, it's in System Preferences > Network > Wi-Fi.
There's the "Ask to join new networks" checkbox which you might want to un-check. Then under "Advanced > Wi-Fi", you might want to remove your neighbors' networks from the Preferred Networks list, and perhaps un-check "Remember networks this computer has joined", and perhaps check "Require administrator password to: [√] Change networks".
Even if you had never even looked for these settings and thus left them at their defaults, your Mac wouldn't have joined those other networks unless at some point you told it to (either explicitly, or by picking it from the list presented when your normal networks weren't found).
By the way, your network and your neighbors' networks all have unique names, right? It's not like you've all left your networks with default names like "Linksys" or something silly like that, right? If you haven't given your network a unique name, fix that first.
Have you tried turning those networks in your 'list of available networks' to manual, so that you automatically connect to something else first? – Rhys Gibson – 2012-06-08T05:45:34.020
From where do I do that? – meetpd – 2012-06-08T05:46:16.520
haven't got a Win 7 box in front of me, but in your control panel, select 'manage wireless networks'. From there you should get an option to change the order of the preferred networks by dragging them around – Rhys Gibson – 2012-06-08T05:49:03.007
Your neighbor was correct to ignore you. It's his right to run an open network if he wants, and there's no reason it should cause any problems for anyone else. It's good that you're trying to learn how to configure Wi-Fi on your clients. – Spiff – 2012-06-08T11:15:39.683