Sometimes it's really need to killing expdp or impdp processes because wrong import or export, or heavy load on database or many other situation.
1) you need to identify job name, which you can view in data dictionary views:
select * from USER_DATAPUMP_JOBS; select * from DBA_DATAPUMP_SESSIONS;
2) you need to attach that job find in above views:
impdp system/***** attach=test_dump(job_name)
3) stop job with command:
STOP_JOB=IMMEDIATE
That's all.