How can I kill a process in Linux when kill -9 fails?

I have a process (actually a pair of them) numbered 1234 which uses my NVIDIA GPUs. I want to kill it. I tried kill 1234; I tried kill -9 1234 I tried kill -KILL 1234 - no effect. Tried Ctrl+C and Ctrl+Z in the terminal session where the process is running - no effect. I tried killing as root - no effect. Or rather, there is some effect: The process state is no Z(ombie). But it still seems to use ~100% CPU. Can I "kill it harder" somehow? Notes: * The process is interacting with the CUDA driver. Trying to tell my GPU to power down then power up (using nvidia-smi) also has no effect. * Power-cycling the machine works :-(