If you want to hide the Minimize and Maximize buttons you have two options:
The easiest way to hide the minimize or maximize buttons is to set the HideMinimizeButton and HideMaximizeButton properties respectively. This is demonstrated in Example 1.
Example 1: Hiding the Minimize and Maximize Buttons
<telerik:RadWindow xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation" HideMaximizeButton="True" HideMinimizeButton="True" Header="MainWindow" />
Figure 1: Result from Example 1 in the Office2016 theme
By setting the ResizeMode property to CanMinimize only the Minimize button will be visible. The resize button will get hidden and the resizing will get disabled. By setting the ResizeMode property to NoResize both the Minimize and the Maximize buttons will get hidden, the resizing will also get disabled.
To learn more about resizing the RadWindow read here.