One of the coolest new features in Server 2008 R2 and 2012 is the ability to recover deleted Active Directory objects. Your forest functional level must be at least 2008 R2 in order to activate this feature. Server 2008 R2 introduced the AD Administrative Center which provides a nice GUI to restore deleted objects after activated. However, eventually one might want to permanently empty the contents of this recycling bin. There is no way to do this via the GUI. The following PowerShell line will do just that.
Get-ADObject -Filter 'isDeleted -eq $true -and Name -like "*DEL:*"' -IncludeDeletedObjects | Remove-ADObject -Confirm:$false
If you want to see what it is going to remove, only use the Get-ADObject section of the command prior to the pipe. Remove -Confirm:$false if you want to see and confirm each deletion individually. Only run this command in the form seen here if you are absolutely sure you want to empty the trash!