I use both those sites many time daily.
No matter how many forum modifications and anti-bot measures are used that still does not stop the human spammers.
You are correct that monitoring and policing by forum members is extemely helpful. Each post has a button in the upper right corner where you can report the post. Moderators get an email for each reported post, so we can react quickly.
Please, if you see something that is spam or you think might be spam, don't hesitate to report the post.