How can be "Export to Excel" button (or option) disabled within lists in SharePoint Online?
I have tried disabling "Use Client Integration Features" in the definition of the permission level, but the option remains there for the user.
Disabling "Use Client Integration Features" should disable the button, but not hide it. As others have said, to remove the button from the Ribbon completely you would need to alter the CSS of the page, but this may not even be possible in the Modern Experience.
You will have to locate the CSS class responsible for displaying and then you can override the class with
this would do the same. You can add any html web part and add this style sheet in that HTML code , Hope it would help resolve.