The goal here isn't to ban people from posting rumors, it's to get people to think about what they're posting and not just regurgitate anything they hear.
For example, here are some simple guidelines that make a rumor posting vastly better:
1.) Generally speaking, do not post negative personal rumors about staff on GAF, and by that I mean things like someone getting fired or forced out or whatever. If you feel strongly about your information, e-mail a site like Kotaku or Polygon with your evidence and let them handle getting additional confirmation and also commentary from the person the rumor is about, and decide whether or not it's worth posting. Now, if it's something like "Oh yeah, they left to found an indie studio, get excited!", I'm not really concerned about that, but strive to not do anything libelous or disrespectful.
2.) In case this isn't clear, it is okay to post rumors about a bunch of people being laid off (not fired, but explicitly laid off) from a studio. We have allowed this in the past and will continue to do so in the future. Please really try to make sure you're certain about this first though. Similarly, if you see a bunch of staff members mentioning they were laid off on twitter, obviously that's fine to post about.
Now, for rumors that aren't about staff:
3.) Never overplay your hand. Only share what you know, and if you wish to make any additional speculation, clearly mark it as speculation. Make sure to state your level of certainty about the core rumor as well. If you've heard that something "has a chance of happening", it's totally different than hearing that it "will happen". This also applies if different parts of the rumor have different levels of credibility. For example, if you've heard that Naughty Dog is definitely making a space shooter and heard that it might have a budget nearing $75 million, make sure that you present the budget as a less certain fact than the game existing. You should explicitly make these kinds of distinctions to the best of your ability.
4.) Ask yourself "Would the person who told me this actually know this information?" Let's say you have a rumor about a game having multiplayer. If your source is a QA person working on that product, they probably do know that information. Let's say your rumor is that a senior staff member was fired (which you shouldn't be posting anyway). If your source is a QA person at a 300+ person studio, chances are they actually don't know that for certain. If your source is an external QA person who doesn't even work at the studio, you should be extremely skeptical of the rumor even if you personally trust the person, since they could easily be hearing gossip that was presented to them as fact.
5.) Ask yourself "Does this rumor make sense?" If your rumor is that Naughty Dog is making an iOS game about farting cats, even if your source is someone at the studio, you might want to think if they're playing a joke on you. Game studios generally aren't water tight, especially if you know what information you are looking for. Check their job postings and public statements and linkedin resumes, and see if that adds up with the information you've heard. A rumor sprung up that Visceral was making a Battlefield title.
It was really easy for me to find buckets full of information supporting this. Chances are you can find at least one thing confirming your rumor before you bother to post it, even if you don't share what that thing is in order to protect your source. There are obviously rumors where this isn't true, but it may be worth noting in your post that you couldn't find external evidence confirming or even hinting at this.
6.) Before you hit post, ask yourself one last time "After checking all this information, do I actually believe this, or at least find it notably plausible?" Chances are that if you've completed the previous steps, by this point you do, but it's worth checking one last time and deciding if you want to do a bit more research first, especially if it's something farfetched.
