1. Go to Commands > Manage post types
  2. Find the post type you would like to exclude from search and click the "edit" button to the right
  3. Click on Post display settings and scroll to the bottom of that section > Post behavior 
  4. You'll see a check box labeled Exclude from search - check this box and save

Note that this only excludes posts of this type from the site search, and not from search engines like Google, Bing, etc.