The window has CanClose property but has no CanMinimize which in my opinion is terrible. We will consider adding CanMinimize property but I can't give you an estimate.
As a solution I recommend you to create a project with the RadWindow and open it in expression Blend. It's quite easy to edit the window's template there and simply remove the minimize button. I'm attaching an example project with the window for office black theme modified. I know It's quite a lot of xaml styles.
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