Edit group to not allow student group members to create forum
1. In the Course Management section of the course menu, click Control Panel, "Users and Groups," and then Groups.
2. Hover over the name of the group, click the down arrow that appears at the end of the group name, and click Edit Group.
3. Scroll down to the Tool Availability section and in the Discussion Board section click the button next to "Do not allow student group members to create forums."