Replying No to an RSVP is handled via Splash Forms. To allow guests to RSVP no:

1. In your Event dashboard select the Registration Form tile.

2. In the Form Editor click on Add Question in the bottom right. 

3. In the default Template Questions tab of the popup find the "Are You Attending?" question, click on it, and click on the blue Add to Form button at the bottom. 

4. Save your changes, and now when your guests submit their RSVPs on the event page they will be able to check whether or not they will be attending.