In Disco, jobs not purged still use some disk space, for temporary data. This can lead to fully using all the disc space in your cluster. I have been there, and it is not fun I promise you.
You can setup some purge policy (a job might purge itself after completion, for instance), but if you need to quickly clean up all jobs, this snippet will help. It will purge all completed jobs, successful or not.
for job in $(disco jobs); do disco events $job | grep "WARN: Job killed\|READY"; if [ $? -eq 0 ]; then disco purge $job; fi; done