Yeah, I gotta say if this is a new rule from PW, or even just a retroactive one that's starting to be enforced now, it's a bad one. The Off-topic forums are.... for off-topic conversation, and I'm not seeing anything in the agreement I checked when I signed up that says I can't discuss non-trek properties.
The closest we get, is here, in the Rules of Conduct (I read through the EULA and the TOS and they had nothing, save for a clause saying you guys can do whatever you want with no limitation- but my lawyer friend assures me that's there to protect from lawsuits, not because you're actually going to abuse it)
(b) Rules Related to Interaction with Other Users. Communicating with other users and Cryptic representatives is an integral part of the Game and is referred to in this Agreement as "Chat." In addition, we and our designees host message boards, blogs, online discussions and other forums found on the Site (collectively, the "Forums") that permit you to interact with other users. When engaging in Chat in the Game, or otherwise using the Game, or when participating in the Forums, you may not:
(i) Transmit, post, offer or solicit (A) any content, information, images or language which, in the sole and absolute discretion of Cryptic, is deemed to be offensive, including, without limitation, content, information, images or language that is unlawful, harmful, harassing, fraudulent, threatening, abusive, libelous, defamatory, vulgar, obscene, hateful, sexually explicit, or racially, ethnically or otherwise objectionable, nor may you use a misspelling or an alternative spelling to circumvent the restrictions set forth in this Section 1(b)(i)(A); (B) any content, information, images or language that infringes our or any third party's intellectual property or other rights;
I will note that the Rules of Conduct and the Terms of Service don't actually give us license to discuss this game without reprise- in fact, the ToU does have a similar line to the above about not infringing on your or any third party's intellectual property or other rights. It also says that people running fansites have to stick up a disclaimer- but it doesn't actually give them license to run those fansites.
Since these threads are clearly not offensive, except in perhaps the vaguest amount of moderator arbitrarily assigned excuse making, I can only assume we're using the latter option- that of copyright infringement.
SInce the agreements we as users have assigned apply equally to Cryptic's own material and any third party material by Cryptic's own wording, um....
Basically if you go ahead with banning all non-trek discussion without first changing the EULA/ToU/RoC/PP all discussion is copyright infringing, and therefor must be banned.
You really might want to look into that.
EDIT: And before you try, there's nothing in these about how word of Cryptic employee on the forums supercedes or clarifies anything written in the aformentioned documents- it has to be in the documents, or it's hearsay.