Thank you for reaching out to the Community!
That is a great question! With our RSVP action block you can export groups of people who say yes, people who say no and people who say maybe. You can then re-import those groups back into your Constant Contact account and add them to their own list. That said, through the RSVP action block there is not a way to easily send just to folks who did not select one of those three options. Essentially what you would have to do is export the report for all three of those groups, add all three of those export files into their own list the remove that list from the original list that you sent the invite to. Click Here for some more information on the action block reporting.
All of this said, this process has been made simple with our event promotion and registration tool. This allows you to send an email invite, and then send a reminder to people who registered and another email to non-respondents. Click Here for some more information on event promotion and registration emails.
I sure do hope that this helps but please let us know if you have additional questions. Thank you for using Constant Contact, have a great day!