With unkillable process a simple reboot won't do much.
Because the system will try to safely kill all process before initiate the reboot, but when you have a zombie process, the hard reboot is the only solution.
To initiate a hard reboot remotely, we have what we call "Magic commands".
Becareful ! This will kill all the running process without save, all non saved data will be lost !
echo 1 > /proc/sys/kernel/sysrq echo b > /proc/sysrq-trigger
In short, the first line will enable the SysRq access on the kernel and the second line will initiate the "hard-reboot" by the SysRq.
If you want to completely shutdown the server, you can do this.
echo 1 > /proc/sys/kernel/sysrq echo o > /proc/sysrq-trigger
If you want to keep the SysRq enabled all the time, add this to your sysctl.conf
kernel.sysrq = 1