OG: how to create a node INSIDE a group?

I created a content type and set it to be a groups content. Now when I create a node of this type its allowed to select a group to where the node be bound. But I don't want users to be allowed to do so - if they ARE on the group's pages (viewing other nodes of the group) they shouldn't be able to select group again.

This is not only the usability issue. Actually, I connect groups to taxonomy terms and want every node to be created as group post to be connected to exact same terms or to subset of them. [I'm yet to determine how I can achieve this and planning to create separate topic about this :)] Thus when a node creation form is first opened to a user, a group must be already known and the list of possible term values which depends on the group is filled correctly.