Instead of advertising to the general public, you should probably make it invitation-only. Collect email addresses of the people who've been attending and let them know notifications will be sent via email. Any new members get invited by current members and approved by you or a committee before being allowed to attend.
I wouldn't take legal action at this point. I would just reply to her RSVP with an unemotional message saying she cannot come. "There are no seats available at the event. Your RSVP has been denied/cancelled" or some shit like that.