It makes it a lot harder if they actually take the trouble to register and then post and you will never completely stamp it out of course. But let’s go through the options….

1. We would aways recommend moderation of posts and yes – you can set it up so that the first post needs moderating and subsequent posts will need moderating if approval is not given. To save repeating things there is an FAQ in the FAQ > How To section (right hand sidebar) of the codex that explains how to set moderation up.
2. If you allow non-registered users to post on your forum then I would recommend our captcha plugin. But this plugin is also useful if you have automated bots leaving spam posts because it will catch them and reject them.
3. I would strongly recommend the use of our Admin Bar plugin. This allows Admins (and moderators if you give them permission) to review all new posts in one place – to delete, approve, load the topic and even reply without having to load any topic pages. 
4. If you do activate our Admin Bar plugin then go to the options for it and turn on the setting to parse your posts through the Akismet service. Any spam posts found will then be presented to you along with an option to delete all posts from the same user.
5. We also have a Ban plugin available and you can ban by IP address but of course dedicated spam posters know how to defeat that easily enough.
6. Finally I would go to the forum admin > options > content settings panel and think about the options to refuse duplicate posts made by members and/or guests. And also, if these spam posts tend to be the traditional one with lots of links, then consider limiting how many links a user can include in a post. The main problem with this, of course, is it applies globally to your wanted users as well.

I hope these help…

We have tried a lot of different plugins and such to deal with spam registration and posts, wang guard, akismet, captcha. The best thing we have found was cleantalk, it costs 8 dollars a year and we went from 50 spam registrations a day to two in the last month that slipped through. Highly recommend it.

http://cleantalk.org/

Is that not just for bot registrations? As opposed to a real human registering? We found just putting our captcha plugin on the WP registration form sorted bots out…

I believe both, when we were getting hammered with registrants I would google the ip address and e-mail they registered with and 99 percent would show up on cleantalks blacklist, even those that had made an initial spam post on the forum. I tried the captcha plug in from here on the registration page but it never recognized the picture as being correct when I tested it.

yes there was a small bug at one time – all fixed up now though…

hmm, have to give a shot again then

been working well for some time… let us know…