How can I prevent an extension from loading into a version of Arcview that the
extension isn't supported on?
Some avenue classes and requests do not work in all versions of Arcview. This
most likely prevents the extension from working in versions that cannot handle
these classes or requests.
Basically, you can only keep the extension from loading the user interface
components. The accepted convention is to detect, in the install script, whether the extension interface should be loaded, and if not popup a message saying that it loaded but cannot be accessed. Since there's no way to access the interface, the scripts cannot get called. You should unload the extension as some errors may occur.
The best way to handle this is when the extension is installed. Use an install program and check the registry to see what version of ArcView is installed. If you are not using Windows, then check to see if a tools folder exists in $AVHOME--this was added at AV31 and its presence is a good tip-off.