If you want to remove all the light from a particular source, you can use the Restrict Lighting to function for the light. It's on the General tab/section of the properties panel.

If you want to control the specular and diffuse component individually, we offer a plug-in called Lighting Control in the Shader Plus package that lets you adjust Specular and Diffuse contributions of individual lights as part of the object's shader. You can get more details at

http://www.digitalcarversguild.com/plugin.php?ProductId=13

Regards,

Eric Winemiller
Digital Carvers Guild
Carrara and LightWave plug-ins