Hi Uberhund and welcome to the forums.
The .NET Micro Framework makes it impossible to assign a pin twice, to prevent some issues. Also, the garbage collector doesn't free up the pins.
Therefor you can use the Dispose() method on the OutputPort class.
You could also make the ledOnBoard static, or a global member of your class. I have the feeling that's what you want to do.
Thanks for the welcome and excellent response, Stefan. Many thanks.