Open powershell (doesn't have to be admin) and run the following:
<code>
PowerShell.exe -ExecutionPolicy Bypass
Install-Module -Name ExchangeOnlineManagement -Scope CurrentUser
Connect-ExchangeOnline (enter your O365 admin credentials)
Set-UserBriefingConfig -Identity someone@somewhere.com -Enabled $false
</code>

To check a user
<code>
Get-UserBriefingConfig -Identity someone@somewhere.com
</code>