Recently the Kdenlive forums seem to be overwhelmed by spam, bumping up old threads (sometimes actually digging up useful forgotten topics...) with meaningless posts.

Quite a lot of these spam posts aren't even bots, so captchas won't work (and there already is a captha when you create a new account).

Is there a know way to actually get rid of them ? As soon as I see such a post I "flag as offensive", but I don't know how well that works.

I don't really know how "flag as offensive" works. But I assume it sends an email to jbm. And as jbm hasn't been around here for some time it is probably pretty useless.
Maybe some people should get the right to delete such posts.

I think the only way forward for us regular users is to keep flooding the forum with meaningful posts.

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I'd be glad to go around deleting the spam posts.

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Given that this is a Drupal powered website I would suggest installing the Mollom module (http://drupal.org/project/mollom).

Mollom is a free service which helps filtering spam. For more informations see: http://mollom.com/how-mollom-works

I delete the spam when I encounter it - I have that permission. But I had not visited in a few weeks due to busyness.

Could you please give some dedicated users rights to delete spam? Call them deputies and give them shiny stars.
And install spam-stopping plugins as mentioned above?

I can not help with site admin much because I do not have access to the server, know very little Drupal, and too busy with development tasks to learn it. But I will look into how I can let others delete spam too since I do have some extra drupal privileges.

I found the menu item (Moderate) that shows flagged comments to make it easy for me to them. However, I found a fair number of comments that were flagged by some well-known forum members that were not actually spam, so that gives me pause about giving some others access to delete posts esp. since its not really "my site." Just keep flagging away for now, and I will review them. There are quite a few non-forums pages with a lot of spam comments that I need to delete. sigh.

I've noticed that most of the posts in the Download section of the website also appear to be spam.

As much as I like the idea of being able to comment everywhere, the reality is that we can not adequately filter and moderate it. Therefore, I disabled commenting just about everywhere except the forums which we regularly check and which clearly shows us new activity. I think we are in much better shape now.

I think I have detected a new form of spam - this guy "egypt" is copying older posts, to give his posts some seemingly on-topic text, before adding the ad link.
See:
http://www.kdenlive.org/forum/chromakey
(the last message)

We even have spam in our screenshots:
http://kdenlive.org/jordans-p90x
http://kdenlive.org/jordans-p90x-0
http://kdenlive.org/coddy-phillips

I think it is time to bring this up again. I like to read through the forums offering help when possible, but it seems so many of the "active forum topics" are actually spam that look almost like real messages - it is very discouraging. Maybe all links should be disabled or get a better spam filter or more moderators?

I gave LeHomard, geoffrey, ttill, rylleman, Marko, and Weevil permission to administer the comments. You can access flagged comments via the left-hand menu Administer > Content management > Moderate.