I use reCapcha http://drupal.org/project/recaptcha. I think its quite reliable.
Also only register user can add comments.

On Wed, Oct 21, 2009 at 6:20 PM, Jamie Holly <hovercrafter@earthlink.net> wrote:
I used to use Mollom + Graphic Captcha + Bad Behavior. This provided the
best protection, but spam still got through. I finally ended up just
removing Drupal comments on my site and using Disqus. Since then I have
had only a couple of spams get through. The nice thing is that I get
email notifications of each comment and can moderate them from those
emails. If I want a comment deleted just reply and put "delete" at the
top of the message. If you really aren't that concerned with having the
comments inside of Drupal, then this is the best route to go.

http://drupal.org/project/disqus

Jamie Holly
http://www.intoxination.net
http://www.hollyit.net



Tine Müller wrote:
> I haven't done anything yet on my site
> http://www.tinemuller.dk/drupal/ to protect against spam. Is
> http://drupal.org/project/mollom the best to use or what do YOU use?
> M.v.h.
> Tine Müller
>
> http://tine_muller.homepage.dk/
> http://www.findbiblioteket.dk/
>
> 0
>
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Mithun Shitole
BE[IT]
Pune.