If using the Scanline renderer, set the light properties back to normal, and disable volume shadows if you're using them (use Ray Trace). If it still has that noise, you could always share the file and I'll play with it.
As for ART, all lights cast shadows 100%. So your best bet would be to add multiple lights so that it'll reduce the darkness of the shadows. The additional lights don't have to cast shadows, and can be placed in the exact same orientation as the shadow casting lights. Then adjust the intensities of all the lights to get the results you want. I don't play with ART much (read:never), so it's possible there are better solutions.
