To block all the results from a specific source, click on:
Edit query > Sources > Turn off sources you wish to block mentions from...
(Certain source is turned off when the button next to their name is grey)
If you wish to block certain Website, Facebook profile, Twitter profile, etc., follow the next steps.
Example of blocking a certain website:
Edit query > Sources > Websites > All websites except... (write the domain of the website you wish to block; for example. nytimes.com)
To include mentions only from a specific source:
Edit query > Sources > Turn on sources you wish to receive mentions from...
(Certain source is turned on when the button next to their name is blue)
Keep in mind: When you edit your query, the results from the last 3 months will adjust to your new setup, while results older than 3 months will stay the same as your previous setup.