It doesn't matter if you paid for it or not, and the EULA is not relevant because it's for the game itself, not the forums (I don't think it even mentions the forums).
Here's the thing, posting on the forums is a privilege, not a right.
You can have your account banned for any reason, or no reason at all. It's completely legal. It's even legal for him to say he won't ban you, and then turn around and ban you. It might come as a surprise to you, but it turns out lying isn't illegal!
Just because forum accounts are now tied to something you paid for doesn't mean that you are now magically entitled to keep them. The forums are still free to sign up to and buying the game doesn't mean you have to get a forum account. It's optional.
See, when you buy the game you're just buying the game, you're not also buying a forum account. The game just happens to also come with access to the social and support section of the website. It turns out that if you abuse this you can be denied access.