When we have multiple tenants in sharepoint 2010 and have different identity provider, in people picker you might see all the trusted provider being listed in the result of "All Users".
We can disable the “All Users” from people picker via the below powershell script.
$cpm = Get-SPClaimProviderManager
$au = get-spclaimprovider –identity "AllUsers"
$au.IsEnabled = $false
$cpm.Update()
