Yeah, this app I want to make it a little bit crazy since envolves API, blockchain, and Stellars! :( But just asked a company and some freelancers for "how much".

No. Maybe filtering the domain with a firewall (or Cloudflare) can help. But formspree and recaptcha only detects a spam bot upon clicking "send".

Formspree have this "display: none" field. So when a spambot tries to send you a message, it discards the message. Maybe an implementation similar to this? Or use "Recaptcha", the new one it's not annoying :)