If you delete an Office 365 Group, it will be set to deleted status for 30 days, then be fully purged.
In order to delete with immediate effect, run these CMDLET lines;
#Install Module if not already
Install-Module -Name AzureAD
#Connect with an Exchange Admin, with Exchange Product license. Or Global Admin
#View what groups exist
#Take the GUID from Get-AzureADMSDeletedGroup and remove
Remove-AzureADMSDeletedDirectoryObject –Id e4a7f376-7aeb-4239-98e4-c0369e53f49f