In order to block out the road underneath the road shield you need to select a road shield symbol that is filled instead of only the outline. Then assign a fill color to the background road shield symbol. For example, the ESRI Shields font contains characters for both the filled and outlined pentagon shape.
Select the filled pentagon character and then assign a fill color and a stroke weight and color.
As for your question about the symbol being off-centered - perhaps the character in your custom font was created off-center so it's not lining up by default. You could try adjusting the Offset Distance values to move the background shape to the desired position.