Welcome to the Make community!
You can’t do it reliably, especially not without making it more difficult for human users.
Captchas can be easily solved by bots, and is not as effective as you think.
One way to stop bots is perhaps implementing a payment system, even $0.10 will do.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!