This can be accomplished in a couple of ways. 

If you want to exclude an item temporarily:
  1. Open KB.
  2. Hover over the item you want to exclude and click the "No Symbol" exclude icon to the right. If you hover over the icon, it will say "Exclude Videos", for example.  The item text will have a strike through on it and all items in that category will not show in the search results.
    User-added image
  3. Alternatively, you can just unmark the item (so the check is not present).
If you want to exclude an item for all future searches:
  1. Complete the steps above
  2. When your search configuration is set, select the Settings icon (3 bars stacked vertically in right corner of image above) and choose Save Filter. This will be the default filter used on future searches.