Remove or Redirect Users who go to www.domain.com to domain.com
If you're using Apache with your setup, this is built into the .htaccess file that comes with the distribution of Drupal. Look for the following code and follow the instructions in the comments:

  # To redirect all users to access the site WITHOUT the 'www.' prefix,
  # (http://www.example.com/... will be redirected to http://example.com/...)
  # uncomment and adapt the following:
  # RewriteCond %{HTTP_HOST} ^www\.example\.com$ [NC]
  # RewriteRule ^(.*)$ http://example.com/$1 [L,R=301]


You'll have to replace example.com with your own site's domain.