Hi Andrea,
Thanks so much for reaching out!
Just to let you know, we are limited by our Support Policy in how much we can help with customizations, but I will try to point you in the right direction.
We don’t have a guide ourselves to implement reCAPTCHA for RSVPs, however, you can customize the RSVP form by following our Themer’s Guide and copying the src > views > tickets > rsvp.php file into your theme from the Event Tickets plugin.
Additionally, let me point you to the reCAPTCHA documentation as reference > https://developers.google.com/recaptcha/
I hope this helps!
Thanks,
Jaime