Captcha can be enabled for this home server. This file explains how to do that. The captcha mechanism used is Google's ReCaptcha. This requires API keys from Google.
Requires a site/secret key pair from:
https://developers.google.com/recaptcha/
Must be a reCAPTCHA v2 key using the "I'm not a robot" Checkbox option
The keys are a config option on the home server config. If they are not
visible, you can generate them via --generate-config. Set the following value:
recaptcha_public_key: YOUR_SITE_KEY
recaptcha_private_key: YOUR_SECRET_KEY
In addition, you MUST enable captchas via:
enable_registration_captcha: true
The ReCaptcha API requires that the IP address of the user who solved the
captcha is sent. If the client is connecting through a proxy or load balancer,
it may be required to use the X-Forwarded-For (XFF) header instead of the origin
IP address. This can be configured using the x_forwarded directive in the
listeners section of the homeserver.yaml configuration file.