You need to modify the control template of ToolWindowStyle in order to remove the close button.
Keep in mind that you will need to edit all resources for ToolWindow for the Summer theme as you use StyleManager theming mechanism. The summer theme resources can be found on your RadControls installation folder, find the Themes.Implicit folder inside and select the Summer folder.
The approach is demonstrated in the attached project.
I hope this information will be helpful.
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