Spam:
Repeated useless posts, especially if they are in the same thread.
"Thank you" has a lower spam value than quoting yourself for instance. Nobody should get banned for saying thank you, but you get the idea.
If you cause the moderators a lot of trouble, THEN you should be banned, not if you post one or two "more or less" useful posts.

I hope you get what I meant with that.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram