I needed this for a side project and i'm placing this script here. It gives a baseline to unmanage a node via PowerShell and SWIS without the need to install the SDK. The baseline can be modified/imported into any current POSH script.
I've used this in automation scripts so that it doesn't alert me on a node while i'm performing an automated task.
NodeID: You can find the NodeID when you browse to that host and look in the URL. I'll be "N:#"
Duration: Time to unmanage node. Examples are provided in script for duration in minutes, hours and days.