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'.
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Set-TridionCoreServiceSettings -Version 2013-SP1
$client = Get-TridionCoreServiceClient
$client.DecomissionPublishingTarget("remove-me-tcm:0-1-65537")
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.