Zac's official response:
It's really disappointing to see players actively griefing other players. The fact of the matter here is that Mobius is a private group where players decide to come together and play in a specific way. The idea that someone decides they want to be disruptive without any real reason or gain is a shame.
However, "griefing" in itself isn't against the rules. There is no evidence that I can see that suggests that the players in question altered the game in any way in order to gain an unfair advantage. In addition, groups are made in a way that allow players to join together and play in a way they want. However, it is the admins of those groups that manage which Commanders have access to the group and which do not.
It is something that David Braben and the developers are aware of though. Only recently I heard David answer a question on this subject in a community interview and I know that it's something that has/is considered by the developers, and I know that creating a friendly and enjoyable game experience is something that is taken very seriously.
I should also mention, the offending players have been removed from the Mobius group. I know this because the Mobius admins got in touch when there were some technical difficulties with managing their group permissions to remove these members from the groups. The members were successfully removed and the technical issues have been resolved.
On a personal level, I must say, on the most part we see community members taking part in PVP or roleplaying piracy in a way that works for everyone. If a trader is stopped and asked to drop their cargo, they feel better for the experience and not having to rebuy their ship - That player is more likely to continue to trade. The pirate can then plunder another day.
It isn’t piracy if a player deliberately tries to ruin another player’s experience. It is unfortunate to see players trying and taking pride in this kind of experience.
My apologies to any posters I may have disagreed with over the previous months- I was wrong, you were right, griefing is not against Elite Dangerous' rules. In fact, it appears that it's not only tolerated, it's so important to Frontier that they're willing to put the amusement and entertainment of a few griefers and gankers before the clearly expressed wishes of almost 20,000 group members- although they're 'disappointed' that those griefers and gankers chose to act that way. Thank you for the clarification, Zac.