Can login as root user on Debian or Ubuntu/RHEL/CentOS Linux based system. I need to log everyone off (all ssh users) to install new kernel and/or hardware. How do I do this on Linux? What is the best way to logout ALL USERS remotely over the ssh based session in Linux like operating systems?

You need to use the following commands:

a] who or w command – Show who is logged on and what they are doing.

b] pkill command – Kill user session and forcefully logout of the system.

c] shutdown command – Arranges for the system to be brought down in a safe way.

Examples

Use the who command to see list of logged in users as follows:
# w
OR
# who
Sample outputs:

root     pts/0        Jul 29 13:53 (10.1.6.120)
tola pts/1        Jul 29 12:30 (10.1.6.121)
tender   pts/2        Jul 29 12:33 (10.1.6.121)

To force and logout nixcraft and sailee user, enter:
# pkill -KILL -u tola
# pkill -KILL -u tender

Alternatively, just try bash and friends kung-fu and save time:

 ### warning must be run as root or via sudo ###
who | awk '!/root/{ cmd="/sbin/pkill -KILL -u " $1; system(cmd)}'

OR

 ### warning must be run as root or via sudo ###
### Safe version :) ###
who | awk '$1 !~ /root/{ cmd="/sbin/pkill -KILL -u " $1; system(cmd)}'

Finally, you can shutdown the system as follows:
# shutdown -h now

Instead of killing all users one by one you can type the following shutdown command with the warning message:
# shutdown -h +10 "Server is going down for maintenance in 10 minute. Please save ALL your work ASAP and logout of the system."