Do you need yo delete a SPO Site collection?
You need to delete it, and then remove it from recycle bin. The next few lines will explain from Powershell command:
$cred=get-credential #This cmdlet prompts you for credentials. Type your Office 365 administration account credentials Connect-MsolService –Credential $cred #connect to your O365 tenant get-spodeletedsite #Get a list of your deleted sites in the site collection recycle bin with get-spodeletedsite | where {$_.url -eq "https://yoururltenant.sharepoint.com/sites/sitecollection"} | Remove-SPODeletedSite #deletes the SC