You can block users from sending you scraps, messages, invitations, friend requests, etc., by ignoring that user. Ignoring someone is a private action, so other users won’t know if they were ignored by you.
To ignore someone, just visit their profile and click ignore on the top of the page, near to his/her name and click on YES button on the message box coming for confirmation. Once you ignore someone you’ll see a red mark over their profile picture, to remember you that you ignored that user.
You can later unblock someone by visiting their profile. Simply click the unignore option. [source]