So you're saying that only certain people are entitled to post on the forums here? I don't see where BellisseriaCommunity has ever implied being a Linden. In fact, their profile specifically states that they are not a Linden. Heck, they even mentioned planning to set up a community center, which means they are paying for premium just as you are.
I see no reason why they cannot post here, especially as this forum pertains to the very community they have a group for. You may not like that they exist, but they have as much right to post here as anyone else does.