There have appeared hundreds of communities on my instance I have never searched. Where do they come from? Is it some spam attack? How do I prevent this, and how do I get rid of them?

  • Unless you want the job of moderating your users you should close registration.

    Barring that you should make sure email verification is enabled, captcha, and maybe even the questionnaire.

    As an admin you can go to the applications icon, looks like a clipboard. Look at all, and see who’s joined. If they’re unwanted or bots, ban them.