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Use Disk Utility. Hold down "option" while opening the File menu to make "Disable Journaling" visible.

Just curious as to why you would like to disable it. Normally, the performance hit is negligible. (Mac OS X: About file system journaling)

edit: note: I wouldn't recommend disabling it on your startup volume. The benefits greatly outweigh any performance hit. Other volumes are less critical (see Apple article above).
 
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