You can prevent Spotlight from showing dictionary results by typing the following into Terminal:
defaults write com.apple.spotlight DictionaryLookupEnabled NO
The same goes for calculations:
defaults write com.apple.spotlight CalculationEnabled NO
Note that for either of these changes to take effect, you need to restart Spotlight. Use Terminal to do so:
sudo killall Spotlight
I'm not sure why anyone would want to disable either of those features, but it is possible.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071210151529570