Open powershell (doesn't have to be admin) and run the following:
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
To check a user
Get-UserBriefingConfig -Identity someone@somewhere.com