Sometimes happen that you can try to stop process in with kill command, but the process still remain. It is because by default graceful signal is used.
To end process for sure you need to use immediate terminations.
Example of used kill process in Ubuntu:
killall -9 Viber
where Viber is the name of the process.
A SIGKILL or a signal 9 kills the process immediately.
A SIGTERM or a signal 15 allows a process to end gracefully.