I tried to decomission a publishing target with PowerShell and Peter's Tridion Powershell Modules.
Running this simple script did not work for me, I keep getting the error
Method invocation failed because [Tridion.ContentManager.CoreService.Client.SessionAwareCoreServiceClient] does not contain a method named 'DecomissionPublishingTarget'..
Set-TridionCoreServiceSettings -Version 2013-SP1
$client = Get-TridionCoreServiceClient
Update: removed the ConnectionType setging from the script above, that has nothing to do with it, the DecomissionPublishingTarget should be available on sessionless clients as well. Added -Version setting cause this is for SDl Tridion 2013.