Yes I've been noticing this happening very frequently in some forums I do visit, mainly phpBB ones.

What 2 of them have done is to ask for the user to write down an image created on the fly, but BACKWARDS.
I haven't seen any of the porn spam posts since this system has been implemented, and I guess that it would need some specific kind bot of bot to get past trough.

I'm not sure if it's completely spam-proof, but seems to hold the spamming to a minimum (if not zero).

Hope it works man, since I get the same kind of weird porn ads as comments in my personal site (which uses Wordpress and therefore allows comments...). And hell, it's ****ing annoying...

You can find some security mods from phpbb site itself! Please have a look at here:
http://www.phpbb.com/phpBB/catdb.php?cat=57
I was also facing such issues with spam bots! Some spammers can broke even visual confirmation!
But after trying "Only Active Members Can Post URLs" I can control the spam posts in my forum!
you can get it from here:
http://www.phpbb.com/phpBB/viewtopic.php?t=464628
thise mode will prevent Guests from posting urls in the forum! also we can configure it thru admin panel and can give some threshold for days and number of posts! So that, this mod wont allow new users to post urls!
Members must be registered for at least Some days ( specify it from admin panel) and have more than Some posts ( specify it from admin panel)! Its working fine for me!
I had specified 10 posts and 2 days as threshold to post the urls!
Usually spam bots will try to post as guests! If its not working, then they will create one new account and will try to post some un ethical links from those accounts! If you specify some threshold, which will be greater than 1 or 2 days, then u can control those spam posts! beacuse, most of the bots will try to relogin using the same IDs after 1 or 2 days!

This is a very similar post to the topic regarding my blog. I decided to go ahead and post it again because it is a different thread and someone might not read the other. The results are the same.

I would like to make an update to my original post. Since I have installed and updated the site, I have gotten no SPAM messages and it looks like a success, at least for now. Apparently the program that the spammers were using against me hit a brick wall with the new additions. This does not rule out future attacks but this looks to be a good start.

Wow I hope this is a joke! Is it possible for a thread telling of how we despise SPAM to attract SPAM bots? Maybe SPAM attracts SPAM?

lol. Bots are surely getting smarter.

These bots don't have serialized member no.s. Have they created more bots in the range (18928-18998) and only few have spammed this thread. Or is it that they wait for some hours to create a new account?

Unbelievable! Bots nowadays manage to bypass all possible protections, including image validation and mail validation! On my forums (IPB 2.0.0) I used to get several bots every day, even though I had both protections active. In the end, I started deleting bots and banning their e-mails, since I noticed that many are coming from similar account, such as @mail.ru or @web.de. Luckily, the number of bots registering has now decreased, but it is still a problem

Yeah image verification is slowly but eventually being broken to pieces, there are bots around capable of getting around quite the number of types of captcha's.

But I guess a combination of things like captcha's, flood posting and what not might atleast alleviate it to be manageable?

Are they posting everywhere or just this thread?

What do you mean?

It sounds like you are just starting off using your forum. I recommend looking at other alternatives before your forum has more members in it. Invision Board is a very great forum software but you need to pay for it. They have annual subscriptions with support. I recommend using Simple Machines Forum at http://www.simplemachines.org I have been using their forum software for years now and absolutely love it. Support is also great when I run into problems upgrading the forum or adding mods to it. Simple Machines Forum uses image verification and also has an option that requires users to click on a validation link sent to their emails before they can login and post anything.

I was reviewing my logs today and all I can say is wow. There are all kinds of accesses to my blog and forums and I wondered what was going on. I soon realized that it was the automated bot army knocking at my door again. The only difference this time was when they knocked, the bug zapper got them and they dropped like flies. Truth be told I had no idea what was up until I review the logs.