Import in bulk - any way to undo?

You can also right-click on the folders under My Folders and select the option to make them Unmanaged Folders. I don't know if you can do that with several folders selected or if you'll need to do it one folder at a time.

You could also select all of the media you don't want to include in the Organizer's Media Browser, right-click and delete them from the catalog. (They won't be deleted from your hard drive unless you select that option.)

There are a couple of ways to "trick" the program into disconnecting all of those folders -- but I'd have to know more about your directory structure to even begin to suggest it.

Or you can just start a new catalog from scratch.

But why not include all of that media? That's the whole point of the Organizer, after all. To help you manage and locate all of the media on your hard drive. There's no disadvantage to having all that media in your catalog.