In the Office 365 admin center, when you delete a SharePoint Online site collection, it is moved to the SharePoint Site Collection Recycle Bin where it will remain and can be restored for the next 30 days. To permanently delete a SharePoint Online site collection and all of its subsites without the use of the Recycle Bin, you need to use PowerShell. This scenario shows you how to immediately and permanently delete a SharePoint Online site collection.

To begin, you must connect to SharePoint Online. See Connect to SharePoint Online PowerShell for instructions.

To immediately and permanently delete a site collection, fill in the site collection's URL between the double-quotes, removing the instruction text and the < and > characters.

$siteCollURL="<URL of the SharePoint Online site collection>"
Remove-SPOSite -Identity $siteCollURL -NoWait
Start-Sleep -s 20
Remove-SPODeletedSite -Identity $siteCollURL

Run the resulting block of commands at the SharePoint Online Management Shell prompt.

Reference:

• https://support.office.com/en-us/article/More-functionality-scenarios-for-Office-365-PowerShell-8d878ea1-75bf-47cd-af10-2f14f297f12d

In the Office 365 admin center, when you delete a SharePoint Online site collection, it is moved to the SharePoint Site Collection Recycle Bin where it will remain and can be restored for the next 30 days. To permanently delete a SharePoint Online site collection and all of its subsites without the use of the Recycle Bin, you need to use PowerShell. This scenario shows you how to immediately and permanently delete a SharePoint Online site collection.

To begin, you must connect to SharePoint Online. See Connect to SharePoint Online PowerShell for instructions.

To immediately and permanently delete a site collection, fill in the site collection's URL between the double-quotes, removing the instruction text and the < and > characters.

$siteCollURL="<URL of the SharePoint Online site collection>"
Remove-SPOSite -Identity $siteCollURL -NoWait
Start-Sleep -s 20
Remove-SPODeletedSite -Identity $siteCollURL

Run the resulting block of commands at the SharePoint Online Management Shell prompt.

Reference:

• More functionality scenarios for Office 365 PowerShell