I used o3de's default shader ball and StandardPBR material for testing, and I was trying to turn off the IBL's diffuse effect on this mesh. I found that the General Settings "Enable IBL" only switches on/off IBL's Specular result on this mesh, just like the switch "Forward Pass IBL Specular" did.

Can I fully turn off IBL's DIFFUSE effect on a material somehow?

Thanks!