I just figured this out, using the great research available here: http://hintsforums.macworld.com/showthread.php?t=114785
Note that I'm using 2 different definitions of "dictionary" in the explanation below:
The trick is to add a value to the AppleSymbolicHotKeys dictionary in com.apple.symbolichotkeys
Within this dictionary, you want to add a new dictionary for key 70 (that's word at point dictionary lookup) with enabled = NO.
Make sure that 'enabled' is a boolean.
Unfortunately, I can't seem to distill it to a single 'defaults write...' invocation. It would be something like this:
defaults write com.apple.symbolichotkeys AppleSymbolicHotKeys -dict-add 70 "{enabled=NO;}"
but I can't figure out how to specify that 'enabled' should be boolean. As written, this makes it a string, which just doesn't work to disable the keystroke. You can run this command, and then edit the plist and switch it to boolean.. that may be easier than editing the file directly.
This will disable the dictionary keystroke. You can use the information from the link above to rebind dictionary to a different key if you prefer.