Click the edit icon for that role. You will see a bunch of checkboxes relating to the action that user can perform.
To disable adding items to your portal - Under 'General Privileges' look for 'Content' and uncheck 'Create, update, and delete' checkbox option.
I guess the easiest way to do that would be to assign a level 1 named user to the user. Only named users with level 2 are allowed to create content.Levels, roles, and privileges—Portal for ArcGIS (10.5) | ArcGIS Enterprise
In case you don't have any level 1 named users available you can configure a specific role for the user switching all create options off. Configure roles—Portal for ArcGIS (10.5) | ArcGIS Enterprise
Many thanks Anthony and Xander,
We are using Portal for ArcGIS (10.4.1) and there is no plan to upgrade to 10.5, Could you please help by providing a solution for 10.4.1?
You can create a custom role with Viewer template and assign it to the corresponding users.
Viewer—Content consumers who interact with maps and view content shared to them in groups.