[bradleyland]

Changing your sound configuration is useless against anyone who has obtained admin rights on your operating system. Windows, Linux, Mac, or otherwise. If they have root/admin, they can override any setting you change at the most basic levels.

If you're still OCD and concerned about it, install Soundflower [1] and you can easily configure Soundflower as your default input, but not feed anything on to the Soundflower bus, thus making the default input silence. This is, in effect, the same as the author's suggestion of setting the default input to line-in and plugging in a stub; also pointless, as a line-in jack has no ability to convert acoustic wave forms to electrical signals.

I feel less-smart for even addressing this question.

http://cycling74.com/products/soundflower/

[doomlaser]

Beat me to it. Though I find the tone of this article incredibly overparanoid.

I'll also link along http://roguemaobea.com. They produce a line of audio apps that can hijack and route sound in OS X for a range of actually useful purposes. They've got tools for doing timed recordings of system audio, Line in audio, or audio from specific apps, as well as a tool for broadcasting any of these audio sources to Apple's Airport Express.

[JonnieCache]

I'd just like to chime in with another vote for soundflower. It's useful for any number of things, I have recently used it to pass youtube videos through VST plugins to clean up the audio, and to make Cave Story, a somewhat old game, work with my external soundcard. You can also use it as an alternative to those ripoff "record your audio output" programs.

It is made by the fine people who produced Max/MSP.

[unwind]

I understood is as the stub was plugged into the input to avoid having the input automatically ignored due to not having a device connected.

I don't know (no experience) if Macs' analog ports were this clever, but considering the hardware to detect presence in the port is trivial, it wouldn't surprise me so that's how I interpreted it.

[bradleyland]

I've seen a lot of Windows hardware drivers that do this. They detect when you've plugged a device in to an audio port and change the config to accommodate. OS X doesn't do this. If you configure Line In as the default input, the OS will monitor that port, regardless of its state. The only adjustment OS X makes is to remember input volumes based on plug state. So for example, if you plug in a set of headphones, the volume will be adjusted to the level that was set when headphones were last plugged in. When you unplug, the volume level is reverted to the state it was in prior to plugging in headphones.