Add a captcha field to the @@register form for anonymous users to secure it from spam bots.
It depends on plone.app.discussion and uses its captcha abstaction
facilities.
In order to use a captcha widget, you have to install one - whether
by depending on the captchawidgets extra of this package or by installing
plone.formwidget.captcha, plone.formwidget.recaptcha (not functional at
time of this writing) or collective.z3cform.norobots.
Then you have to configure plone.app.discussion to use a captcha widget. You don't need to enable commenting on your site for this to work.
You will need to pin the version of plone.app.users to 2.1.0.
plone.app.users 2.2.x is Plone 5 only.