A vital module for any modern Drupal website that allows commenting, posting or form submissions of any kind is the captcha module. Any dynamic website that takes user submission should implement some form of "human authentication."
I have two sites, this one and my family's and my family's had just started receiving submission spam, both from the contact form and user registration.
The captcha module forces the user to answer a math problem, question or determine what number/letter combination is disguised in an image, my least favorite, although the other methods can be beaten by scripts.
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