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.

Getting keys
------------
Requires a public/private key pair from:

https://developers.google.com/recaptcha/

Must be a reCAPTCHA v2 key using the "I'm not a robot" Checkbox option

Setting ReCaptcha Keys
----------------------
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_PUBLIC_KEY
  recaptcha_private_key: YOUR_PRIVATE_KEY

In addition, you MUST enable captchas via::

  enable_registration_captcha: true

Configuring IP used for auth
----------------------------
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.