While we’re waiting for full support for modern pages and how they work in Office PnP and the SharePoint Framework, here’s how you turn off the banner on article pages – basically setting them to the layout used by the Office 365 Group Team sites.
If you want to turn the banner back on, set the PageLayoutType to “Article”.
# Connect to the site
Connect-PnPOnline https://tenant.sharepoint.com/sites/portal
# List all pages, and find the id of the modern page
Get-PnPListItem -List SitePages
# Change layout from "Article" to "Home" to remove top banner
Set-PnPListItem -List SitePages –Identity <id> -Values @{"PageLayoutType"="Home"}