This arrow button is actualy the minimize button of the RadOutlookBar that comes with the new minimization capabilities. In order to hide it you can simply set IsMinimizable=false
, or if you want to keep the minimization feature you should edit the default style of the RadOutlookBar in Blend, find the button and remove it or collapse it. If you need further assistance do not hesitate to ask.
All the best,
the Telerik team
Browse the videos here>>
to help you get started with RadControls for Silverlight