Hi there and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
Currently, the best built-in way to prevent RSVP spam is to use the Require users to log in before they RSVP option in Events -> Settings -> Tickets
That being said, you can still add reCaptcha to any form. To do that you will have to do a template customization.
My colleague Josh has some great insights on this: https://theeventscalendar.com/support/forums/topic/add-captcha-to-events-with-tickets/#post-1147570
Let me know if that helps.
Have a great day!
Brendan