You can kill it from SM50 itself. Go to transaction SM50 and you will be able to see the background work process which is running that particular job. Select the work process >> process >> cancel with core.
If you want to kill it from OS level, you can get the process id of that work process from "PID" column in SM50 and go to OS level and kill it with "kill -9 <PID>
To completely deactivate a job from running , select the next job occurence by searching its released status and then goto the tab Job--> Released to Scheduled.This will stop the job from running again.
However to terminate an active job,goto SM66 , Edit
>Process>restart after error >yes and then Process>cancel-->with core.