Spam bots flooding static website and filling `<form>` fields

What type of hardware are you using: VPS bought online
What YunoHost version are you running: 12.1.6.1
What app is this about: my_webapp

Describe your issue

I host a static website that contains a simple <form> field.

That form is filled out and submitted by an average of five spambots (maybe AI crawlers) per day.

How can I block bots? I have a pretty strict robots.txt, but I guess nobody respects that any more nowadays.

Related threads

Share relevant logs or error messages

Not relevant

Did you try adding a captcha to the form ?

Here are some open-source self-host options :
https://alternativeto.net/software/recaptcha/?license=opensource&platform=self-hosted