Some time back I wrote a quick how-to on disabling autoinput and autocomplete in Open Office. Libre Office 3 has the same problem: defaults that make the tools harder to use. The instructions there are still essentially correct but the wording on the menus has changed slightly, and I'm working with a fresh install, so I decided this is a good time for an update.
In Libre Office Writer click Tools|Autocorrect Options. Then select the Word Completion tab and uncheck Enable Word Completion. Also uncheck Collect Words, because there is no need to collect words for a feature that will never be used.
In Calc, click Tools and then Cell Contents, and uncheck AutoInput.
[ Note added 10 September 2012: above I should have written OpenOffice and LibreOffice as one word each, not two. Oh well. ]