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