I don’t know if anyone has let you know - but this does exist now in the Sales Plug-In (althought it needs some improved functionality).
If you are in Hubspot and you go to your settings within the Plug-In, there is now a place to enter addresses/domains that you want to exclude from syncing/tracking.
IMPROVEMENT request for Hubspot - I’d love it if there was a way to exclude domains and have it automatically applied to all the users of the plug-in. For example - I would like to exclude our external IT and our insurance contacts (I know I can’t minutely exclude every single little thing - but this would help).