Hello,
please post questions on Cinema 4D plugin development in the "Cinema 4D Development" forum. When doing so, please also use the appropriate tags.
MATERIAL_GLOBALILLUM_GENERATE is the correct ID of that parameter. You just have to set the parameter value of the material you want to edit:
material->SetParameter(MATERIAL_GLOBALILLUM_GENERATE, false, DESCFLAGS_SET::NONE);
please notice that the ID is defined in the millum.h header file.
If you have general questions on how to handle materials with the C++ API you will find information in the documentation:
Materials and Shaders Overview
BaseMaterial Manual
Material Manual
best wishes,
Sebastian