Stating the obvious without adding anything to the discussion is spam.
For example if I were to post in a dead thread something along the lines of
Originally Posted by mimatin
Very good point, I totally agree with you assessment of the topic
That would be spam. I would be warned and should I continue someone would infract me.
Now if were to post in that thread, but instead of just merely stating the obvious, I added addition information to the topic, such as a link to new information, then that would not be considered spam. Although if I posted a link to information that had already been posted in the thread, but I was too lazy to read the posts in the thread before posting it myself, then that to would be spam.
Add to the discussion in a productive way and you will not have to worry about moderating.