If you want to stop receiving mentions from a particular source, there's two ways you can do that:
In the feed click on the "x" on mention and select Block mentions from domain (this action can be applied to any website or social media profile). You will no longer receive mentions from the blocked source.
Click on Edit query > Sources > Websites > "All websites except.".. and then write the URL of the website you wish to remove.
We would advise to make sure you extracted your data in a form of a document or report prior to making a change because by editing query you're restarting the search.