Below is a sample from one of my apps. It sets the behaviours for all of the windows invoked by this page. You can do this on a window-by-window basis by setting the Window property rather than that of the WindowManager.
|WindowManager.Behaviors = WindowBehaviors.Close | WindowBehaviors.Move | WindowBehaviors.Resize | WindowBehaviors.Maximize;
You can do this 'cos the WindowsBehaviours enum is defined using the [Flags] attribute. See this MSDN article
for more details about enums and the [Flags] attribute.
Hope this helps.