You know what's worse than spam? People posting in response to people they think are spamming and telling them off. This is far more annoying than the people who they are complaining about.
As found when we experimented with extremely tight controls on posting in the C07 forum, that what many consider spam is actually quite important in motivation for people making good downloads. As such, even though there is the thanks button, the only sort of posts that should be reported are ones where they clearly have not read the thread, like saying great work when nothing has been released (and the like).
Saying thanks for something isn't spam in itself.
If people start taking a thread way off course, then there's an issue, otherwise what you are considering spam is fine.