To kill any process yo need to first find out the process ID (PID)
For example if you want to kill firfox
#ps aux | grep firefox
This will generate:
username 5365 0.0 0.0 3572 628 pts/0 S 11:00 0:00 grep firefox
Now kill process using PID
#kill -9 5365
where -9 is special Kill signal, which will kill the process.