Using lightMaps and disabling diffuse illumination on lights

Hi, I’m working on a scene to visualize a realistic living room, where most of the illumination in precomputed on lightmaps.
The scene has some spotlights which should generate a specular highlight on many surfaces. The lightmaps already include direct diffuse illumination of those spotlights.

I’ld like to be able to make the spotlight only contribute with specular illumination, because now those areas illuminated by the spots get direct diffuse illumination twice.

Is there any easy way to do that appart from hacking into the MeshStandardMAterial shader?
I used the beforeRender method once to modify some aspects of the illumination.

This is an image of the scene. The spotlights over the table should only contribute with specular illumination

No, this requires a modification of the shader. There are several possibilities to do this. Read the this answer at stackoverflow for more information: