Thank you for your interest in RadDiagram control.
To close the gallery you can try setting the IsOpen property of the RadDiagramToolbox to false. As for the bottom border which contains the open/close button you can extract and edit the default template of the RadDiagramToolbox. In your case, after you have extracted the template you can navigate to the RadButton with x:Name="OpenCloseButton"
and remove the whole border which wraps the button. You can take a look at the Editing Control Templates help article in our documentation which describes how you can extract and edit the default template of the controls.
For your convenience, we have created a sample project and attached it to this reply which demonstrates the approach mentioned above.
Telerik by Progress
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