You can follow these steps to disconnect Google Calendar / Google Drive with Marvin:
Make sure that you're signed in via the Google account used for Marvin
Search for "Marvin"
Click "Marvin: User Interview and Research Platform"
Click "Delete the connections you have with Marvin: User Interview and Research Platform?"
Click "Confirm" to disconnect
You can reconnect your Google or connect to a new Google account at any time via Marvin's Integrations page.