I’ll introduce the response I received from official support regarding how to delete all past forum archives in Google Groups.

Google Official Support Response

By unchecking “Archive messages to the group” under Manage > Information > Content Management > Archive > Options, you can configure not to archive forums afterward, but past archives will not be deleted.

Unfortunately, there is no button to delete past archives in bulk, but by following the steps below to select all topics, you can delete topics relatively easily.

Steps to Delete All Topics

1. Display the group screen.
2. Click the checkbox to the left of the latest topic to select it.
3. Scroll down the scroll bar on the right side of the screen several times to display the oldest topic.
4. Hold down the [Shift key] on the keyboard and click the checkbox to the left of the oldest topic to select all topics.
5. Click [Actions] > [Delete].
6. When "Topic deletion confirmation" appears, click [OK].

That’s all from the Gemba.