I want to modify the SAP Demo News XML Form, so that the Author of the News is always the actual user ($sap_user) and can't be changed by the user. Is there a possibility so set a kind of Read-Only-Access on the textfield? Or is there another way to realize that?
Yes you can do that.
The author of news is the same that created by (property metadata). If your answer is yes, you only must show created by property in the Show or RenderListItem forms. In both of them create a label and assign this property.