Greg is right (as usual).
You have three ways for that.
- The keyword panel on the right: tick the checkbox of the keyword. Right click the keyword name and choose 'Exclude from search'.
- Menu find, by detail. Select the keyword. When the result is displayed, look at the dark grey horizontal bar just over the thumbnails. On the right there is an option button. Choose "show results that don't match"
- On the advanced search bar (magnifier icon on top right) search for the keyword. Switch back to the thumbnail view. Do the same as above to "show results that don't match.