how to find background job and stop them

Two ways:

             If you know Job name, goto SM37 directly-->enter username and job name-->tick all status which showed as check box 

 -->execute--->select items---> menue "job"--->cancel or delete.

 

           If you don't know the Job name, you can go to se11, enter table "TBTCP" to find the job name first, then perform above steps.