title: Automatic spam filtering or flagging for Django or Python?
body: <p>I'm working on a Django-based site that consists mostly of user- 
  generated content: reviews, comments, tweet-like posts, etc. </p>
  
  <p>I'm concerned about spam. Are there any spam filters available for 
  Django/Python? If not, what types of algorithms can be used for automatic spam 
  filtering or flagging? </p>
  
  <p>On a more general note, does anyone know how do major sites like 
  Amazon and Yelp prevent spams in their user-submitted reviews? </p>