Last updated July 27, 2008.
Here's a sample/template e-mail for warning a user about a junk post the first time they do it.
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Subject: Please do not put test messages in the Drupal forum.
Dear offender_name,
Your post "title_of_post" has been deleted. Please do not post such "test" messages in the Drupal forums. We welcome and encourage questions in the forums, but anything else is not appropriate and will result in your access to the site being blocked.
Sincerely,
your_name
Drupal Documentation Team Member | Drupal.org Site Maintainer