Toribash
Yes, spam is 'illegal' as you might say, on the forums. Useless posts usually get deleted and/or the user gets infracted for it. If you see a post that is spam, feel free to report it using the little "report" button below the achievement button.
Yes, that's the correct, the forums actually have quite a few different regularly enforced rules. You can view the general forum rules here.

I would recommend reading each of the board's specific rules prior to posting there.
Spam isn't 'illegal' ingame, it is just annoying / a nuisance, spams up the chat log, and consistent occurrences of it can generally lead to the player having action taken against them.

On the forums, the same thing happens, except with the happening of creating multiple useless threads (aka thread spamming) and then 'useless posts'.
I'm a game developer for other games.
Ping me if you have a query or general question unrelated to this game.
Originally Posted by StarBall View Post
Okay but if i need to make + threads, WITH decent discussions...

that's is "ilegal" ?

Not entirely sure what you mean? Feel free to write in your native language, there is normally someone around who can help.

If you mean multiple threads for questions, it's normally better conduct to condense these into one thread. For example the two threads you recently created here could have been done in one thread with two questions.

However, for the most part, if your threads aren't stupid and don't directly break a stated rule, you should be fine. If you are unsure, prior to creating a thread PM a moderator or anyone who looks knowledgeable regarding forum matters. We all like to help.