Garageband – Remove Dead Space at End of Track

As far as I know, GarageBand doesn't add anything to the end of exported tracks, but it exports as much as there's sound in any track. Therefore, to avoid having extra "stuff" exported, you should always create a loop region and export with that. As you can see in the following picture, I'm at second 47 in the "timeline", however, I've created a region (the yellow bar on top) that starts at second 1 instead of 0, therefore creating a 46 second clip.

The exported version can be seen in quicktime with a length of 46 seconds, without any extra silence or sound.

In order to create such region, you have to activate the "repeat/loop" (next to the Play button in GarageBand). That will create an extra space below the Beat Bar (where you move the Play Head). When you hover your mouse over that small region, the cursor changes. You can click and "drag" to paint the yellow zone. That's the zone that will be repeated over and over.

Always export with a repeat/loop zone created and Garageband will only export that.

NOTE: Logic Pro/Express behaves the same way when creating bounces.

There's a draggable cursor at the end of the timeline on the upper right corner. If it is beyond the end of your sound on your longest track GB will include that much silence. Drag it back to the end of your longest track, or leave however many seconds of silence you want.

Update 2020 - What about iOS?

I don't use macOS anymore but if you want to do this on iOS or iPadOS, if I don't incorrectly recall, your only choice is to have a region that encompasses the part you want to export, tap it (to select it), then tap it again (to see a floating set of buttons), and pick "Loop". I don't know/think there's a way to do the same "playback region selector" on iOS. I may be wrong, but this question was about the OS X version of GarageBand so I don't know more :)

You cannot change depth and frequency of the recorded samples, even when changing the input settings in the Audio MIDI Setup.app (this is the way to change playback and recording settings for sound devices). The default values for Garage Band are 44kHz, 16 bit, stereo. They are kept in AIFF format. You can explore them by right-clicking your garage band project and choosing Show package contents. The recorded files are kept in the Media folder.

Edit (added Logic 9):
In Logic 9 you can change the sampling rate (frequency) from the Project Settings window, Audio tab. Explained in the Logic 9 Manual - Setting the Sample rate. To change the bit depth you should go to Preferences -> Audio and check/uncheck 24 bit recording (It's checked by default). It's covered in the Logic 9 Manual - Setting the project bit depth.