Thank you for writing.
It appears that the left column is visible and this is where the line comes from. You can hide it like this:
((RadDropDownMenuElement)radDropDownButton1.DropDownButtonElement.DropDownMenu.PopupElement).LeftColumnElement.Visibility = Telerik.WinControls.ElementVisibility.Hidden;
I hope this helps. Should you have any other questions do not hesitate to ask.
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