RSVP means that you want a response, whether the invitee is attending or not. Thus, if you have a link in your e-mail saying “RSVP Now,” the link should take me to a form that gives me an option to say that I’m not attending. Particularly, if your e-mail was a mass mailing, it would make sense to include options like “Not Attending, but interested in future events” and “Not Attending, please do not send me further e-mail”.
If the intent is to only get a response if the person is attending, then sadly, you can’t use an easy shortcut. You can say something like, “Click here to reserve your spot,” or “Let us know if you’ll be there.” If the intent is to get a response only if the person is not attending, the polite phrasing is “Regrets Only” or “Respond with Regrets Only.”
The More You Know…(star whoosh)