You can't really prevent the items from being rendered as links: you could use templates (templates render DIV elements), but that's as far as you can change items.
On a related note, if using PanelBar is required for its collapsing capabilities, why not use content template with RadioButtonList inside? (See attached example.)
Of course, if your scenario does not allow for the above solution, or PanelBar is required for other reasons and links (A elements) are note an option, templates are only solution.
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