It would be very helpful for us if you could provide some markup of how to replicate the issue. Meanwhile however I can only assume about the reason causing this appearance. It could be, for instance, that you apply some custom styles. You could try though to alter the ButtonSettings
of the RadListBox control as described in this demo
and see if this will work out for your scenario.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed