how can I kill all the processes belonging to an user.
I need it because I can't see a process initiated by a user and thus unable to kill it.
To kill a user's process, you must be that user or be root. Under either circumstance you would be able to see the user's processes. Why do you think the user has invisible processes?
ps -fu joe
will show all of joe's processes.
you can also use pkill -u <USERNAME> this will kill all the processes.