I want to get all the search results from the tenant except from a specific result source in SharePoint Online. Is it possible?
There is no way to remove a specific result source from all results directly. You can try to change your query text to contain adverse conditions with the specific result source.
Or, you can go to Center Admin->SharePoint->search, and click "Remove Search results", then type the URLs of the specific result source into it.
Then go to Search Center, type "*", it will return all results except the removed URLs.