Philippe,
This issue has been resolved in the 3.4.3 release which is available to subscribers of the commercial version of OpenEMPI. This fix along with others will be made available in the next community release later this year.
For know, you can connect to the Postgres database and delete the jobs manually using something like this:
delete from job_entry_event_log where job_entry_id in (select job_entry_id from job_entry where job_status_cd = 2);
delete from job_entry where job_status_cd = 2;