All RadControls are editable in Expression Blend. There, drag and drop a DatePicker control, right click on it and select Edit Template->Edit a Copy. After that, Blend will generate all the styles for you. There you can find the ToggleButton and set its Visibility property to Collapsed. This will hide the toggle button.
Attached, you will find a simple running project. Please mind for the NOTE: comment which indicate the place where changes have been made.
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