First, find the process with ps aux, e.g.,

ps aux | grep theprocessname

… which outputs something like the the following.

USER       PID  %CPU %MEM  VSZ RSS     TTY   STAT START   TIME COMMAND
root  29217  0.0  0.0 11916 4560 pts/21   S+   08:15   0:00 theprocessname

.. the second column is the process id (PID).  Kill it as follows:

sudo kill 29217

.. run ps aux | grep theprocessname again, and verify it is no longer running