Hi,
The solution will depend on the render engine used. In case of
mental ray, Forest Pack includes a custom shader that enhances the rendering of billboards -
Forest Shader, which requires the use of
Forest Material.
If that's your case, I would suggest you to lower the
Self-ilum factor value (by default set to 50%) under
Material - Forest Shader. That should eliminate that undesired effect.
Anyway, as you've already tested, exclusion affected items from light source would be an another suitable option.
Best regards,