`echo 0 > /sys/devices/system/cpu/cpu[0-n]/online` should do what you want.

Even as root, that gives `permission denied` on Pi OS on the Zero 2 W. I'm trying the kernel cmdline option too.

Edit: That works! See: https://www.jeffgeerling.com/blog/2021/disabling-cores-reduc...

Well… I said it should, not that it would. ;-)

On x86 there was an option, i think nr_cpus or something like that some time ago. There was also SMP_KERNEL kernel config option.