Powershell (run as admin).

First time only:

Set-ExecutionPolicy RemoteSigned

To disable clutter for all users:

$UserCredential = Get-Credential

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session

Get-mailbox -ResultSize Unlimited | Set-Clutter -Enable $false

Remove-PSSession $Session

Source: Office365 Community