The only relevant thread I found on WordPress support forums about this was closed to replies, so I’ll make a note of what I found here: removing the ‘lookup’ quicktag button from the post editor can be achieved by commenting out line 196 (WP 2.2) of quicktags.js (which is in wp-includes/js/), like this:
/* document.write('<input type="button" id="ed_spell" class="ed_button" onclick="edSpell(edCanvas);" title="' + quicktagsL10n.dictionaryLookup + '" value="' + quicktagsL10n.lookup + '" />'); */
No need to edit anything else, although I suppose you could comment out the edSpell function too, since after the change there should be no use for it.
Note that at least in my Firefox/Win I had to clean up the cache before reloading the post editor would reflect the change.