If someone sends you inappropriate things, sending in a ticket by selecting your game and then the other category being Player Report or even General will suffice. If you think a forum ban was not fair, then again select your game and choose General or even Account (Of course only if you were the one banned as others can not discuss another persons ban). When the ticket is viewed the GM will see that it pertains to the forums, and that's fine.
If the issue happens in a forum thread/post, then we already have that handy Report Post feature above each post, so it's very easy to report for that.
If you're finding the confusion is in that you want to send in a ticket about forums and so you wouldn't usually choose a game in the first category, it really is ok to just select your game and then send in your ticket about the issue in forums, like some people already do.
Hope that helped~