Can I customize the message displayed to users after they've submitted a response and, for example, hide the 'Submit another response' link with custom CSS/HTML?

While direct manipulation of the 'Submit another response' link with CSS or HTML isn't possible within Google Forms' standard features, you *can* customize the confirmation message. You could include a message that instructs users on how to proceed after submitting if you do not want multiple responses. Further customization (e.g., complete CSS override) would typically require integrating the form with a more advanced platform or using an add-on with more functionality.