Run ps aux to see a list of all running processes. To the right of the process name will be a process number (PID), like 606 or 1125 or 12856.

To kill the program, enter

kill [pid]

If you've still got a problem, then try

kill -9 [pid]

This will definitely kill the program.

To kill a background process or daemon, use the HUP (hangup) parameter, which tells the program to kill iteself, then restart & reread its configuration file:

kill -HUP [pid]