You can just use adhesive tape to cover up the lanes as done here: http://www.techpowerup.com/reviews/NVIDIA/GTX_980_PCI-Express_Scaling/
"This review is made possible thanks to an awesome BIOS option given to us by the ASUS ROG Maximus VI Hero motherboard which allowed us to toggle the CPU's PCI-Express root complex between PCI-Express 1.0, 2.0, and 3.0. Using common plastic adhesive tape to cover up and disable lanes, we also successfully modified the number of lanes available to the GPU for these tests."