In a certain screen I want to validate the entered fields when the user presses the "Enter" key. This works fine with the "On Enter" command. But after data validation, the standard SAP action for "Enter" is performed and it goes to the next screen. How can I prevent this?

Put a "Return" statement in your InputScript after you have validated the screen input. Do not use a "Screen" command in your InputScript. In this case, no "Enter" is performed by the SAP application.