To limit a CPU to a certain C-state, you can pass the processor.max_cstate=X option in the kernel line of /boot/grub/grub.conf.
Here we limit the system to only C-State 1:
kernel /vmlinuz-2.6.18-371.1.2.el5 ... processor.max_cstate=1
On some systems, the kernel can override the BIOS setting, and the parameter intel_idle.max_cstate=0 may be required to ensure sleep states are not entered: