param(
    $url = "https://blah.sharepoint.com/sites/dev",
    $username = "admin@blah.onmicrosoft.com",
    $password = "oopsalmostforgottoremovethis"
)

clear-host

Add-Type -Path "C:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions15ISAPIMicrosoft.SharePoint.Client.dll"
Add-Type -Path "C:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions15ISAPIMicrosoft.SharePoint.Client.Runtime.dll"

$securePassword = ConvertTo-SecureString $password -AsPlainText -Force

$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($url)
#o365-auth
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $securePassword)
$ctx.Credentials = $credentials

$list = $ctx.Web.Lists.GetByTitle("Documents")
$ctx.Load($list)
#http://sharepoint.stackexchange.com/questions/126221/spo-retrieve-hasuniqueroleassignements-property-using-powershell
Invoke-LoadMethod -Object $list -PropertyName "HasUniqueRoleAssignments"
$ctx.ExecuteQuery()

#break permissions inheritance
if(!$list.HasUniqueRoleAssignments)
{
    $list.BreakRoleInheritance(($copyRoleAssignments=$false), ($clearSubscopes=$true))
    $ctx.ExecuteQuery()
}