I missed one spammer today on Indonesian Linux Forum and it posted porn messages across the forum and i was quite late noticing it, since i just came online tonight, but i managed to remove all of it's post and delete it's username to prevent further action.
Budi from Baliwae told me a nice Anti Spam tool which can be used for phpBB and other forums. It's called bbAntiSpam. It's very easy to use and customizable. The installation procedure is very easy. All you need is just upload the files to the server and add one line in one of the file to include those files and it will start working at that time. For phpBB, you can look on this forum, meanwhile, for WordPress, you can find it here. There are a lot more.
The script is free, but it's limited in it's feature. If you want further customization, you will have to buy the license.