If your course will not use ANY groups at all, please follow the steps below:
1. Find the Administration Block and click Edit Settings.
2. In the Edit Course Settings screen that opens, scroll down to the heading Groups.
3. In the dropdown menu for Group Mode, select No groups (by default, every new course is set to Visible Groups automatically).
4. Finally, click the blue Save and Display button at the bottom of the screen.
That should prevent any new forum activity you create from being in Visible Group mode automatically.