First you have to find the process number.
The example tries to find the process number for a process called sixc
surf5:~ % ps -ef | grep sixc
  opid03 24843 24836  3 09:13:19 ttyp4     0:02 /users/specadm/.//bin/hp10/sixc
-t esrf -T esrf
  opid03 24923 24891  0 09:19:19 ttyp6     0:00 grep sixc              


Then try first a soft kill:

surf5:~ % kill 24843


If that does not work you can still try a forced kill:
surf5:~ % kill -9 24843