Have you deleted a site collection hoping to free up disk utilization only to find the deleted site collection lingering in the recycle bin?
Unfortunately, the recycle bin does not provide the ability to delete these site collections.
You can use client-side Windows PowerShell to delete the site collection using the following process.
1. Launch Windows PowerShell as Administrator by right clicking the Windows PowerShell icon
2. Click Run as Administrator
3. Cut and paste the following script into the Windows PowerShell window.
$adminusername = "<SharePoint Online Admin user name>"
$adminpassword = "<SharePoint Online Admin user password>"
$siteAdminURL = "https://<tenant domain>-admin.sharepoint.com"
Import-Module Microsoft.Online.SharePoint.PowerShell –DisableNameChecking
$securePassword = ConvertTo-SecureString $adminpassword –AsPlainText –force
$O365Credential = New-Object System.Management.Automation.PsCredential($adminusername, $adminsecurePassword)
Connect-SPOService -Url $siteAdminUrl -Credential $O365Credential
Remove-SPODeletedSite -Identity <site collection URL> #ex: https://<tenant>eccuorg.sharepoint.com/sites/project1
4. Verify the site collection is deleted from the SharePoint Online recycle bin.
Written and composed by one of our Senior Microsoft SharePoint Architects, Jim Eagleton.