Hey guys, I thought I would update this thread since it came up as one of the top search hits when I tried "expdp kill datapump".

The above answer was correct, but to be explicit, it's referenced from Support.Oracle's
How To Cleanup Orphaned DataPump Jobs In DBA_DATAPUMP_JOBS ? [ID 336014.1] article.

Check for datapump jobs in sqlplus:
TTITLE 'Currently Active DataPump Operations'
COL owner_name FORMAT A06 HEADING 'Owner'
COL job_name FORMAT A20 HEADING 'JobName'
COL operation FORMAT A12 HEADING 'Operation'
COL job_mode FORMAT A12 HEADING 'JobMode'
COL state FORMAT A12 HEADING 'State'
COL degree FORMAT 9999 HEADING 'Degr|(Num|Wrkers)'
COL attached_sessions FORMAT 9999 HEADING 'Sess'
SELECT
owner_name
,job_name
,operation
,job_mode
,state
,degree
,attached_sessions
FROM dba_datapump_jobs;

-- make sure job is in NOT RUNNING status.
Owner JobName Operation JobMode State Degr Sess
------ -------------------- ------------ ------------ ------------ ----- -----
SYS SYS_EXPORT_FULL_01 EXPORT FULL EXECUTING 2 0

-- delete the master table:
drop job sys.sys_export_full_01;