Restrict Access For An Existing Shared Drive

In your Google Admin console
Go to Apps > G Suite > Drive and Docs.
Requires having the Service Settings administrator privilege.
Select Manage shared drives.
Hover over a shared drive, and click the Settings button.
Select Prevent full-access members from modifying shared drive settings to keep people from overriding the default settings for the shared drive.
If full-access members can modify shared drive settings, click Edit to modify any of the following options:
- Sharing outside your organization—Allow or prevent external people from accessing files in the shared drive.
- Sharing with non-members—Allow or prevent shared drive members from giving non-members access to files in the shared drive.
- Download, copy, and print—Allow or prevent commenters and viewers from downloading, copying, and printing files in the shared drive.