How to ban someone from a facebook group?

[Wyatt Earp]

There are a few things you can do:
You can anonymously report messages by clicking on the "Report Message" link near the bottom of each message. You should see this link below the options to Close, Reply to or Delete the message. Facebook will review your complaint and act accordingly.
You can easily block any user on Facebook from seeing or contacting you by entering their name in the Block People section on the Privacy page.
You can limit the people who can find you in searches, which in turn limits who can message you. This option is available on the "Privacy page".