How to stop running job

A Documentum job consists of a dm_job object, containing job configuration, name of method to run, as well as process execution information. The job method is executed on the Content Server (if it's a java method - on the JMS). Once the job is started, there's no 'soft' approach to stop it, you can only force it.
Below are the steps to stop properly a running job, reseting also the dm_job object:
1. Get the process id, by running the DQL:
select a_last_process_id from dm_job where object_name='[JOB_NAME]'
2. Using Process Explorer (or similar tool) locate the process by id and kill it. (Or with a command like: taskkill /pid 1234)
3. Run the API command to unlock the job object:
unlock,c,[job_id]
4. Execute the following DQL queries to clear job execution-related attributes:
a) Reset application that uses/locked the job
update dm_job objects set a_special_app='' where object_name='[JOB_NAME]';
b) Reset current status
update dm_job object set a_current_status='FAILED' where object_name='[JOB_NAME]';
c) Prevent job from being run immediately
update dm_job object set run_now=FALSE where object_name='[JOB_NAME]';

Remember to change the schedule or inactivate the job if you don't want it to run automatically after specified period.
If your job is locked in running state, check here how to unlock it: How to unlock a job in running state.