kill all users session under Linux

You need to use the pkill command which will look up or signal processes based on name. It can send the specified signal to each process.

# pkill -9 -u username

In this example, list all process owned by a user called lighttpd, enter:

# ps -fp $(pgrep -d, -u lighttpd)

To kill all process owned by lighttpd user, enter:

# pkill -9 -u lighttpd