You'll need to go through the filters located on the right side of Mentions center

You can use operators as with regular search so for example you can look for mentions coming from Twitter but not from YouTube then you'll need to search for
twitter.com{AND}{NOT}youtube.com

HINT: you can save such search for later use, just scroll up a bit and use

Your filters will be displayed under Saved filters where you are able to manage them :)

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