When user synchronization is configured and changes are synchronized "DROPPED", sling job nodes under /var/eventing/jobs/cancelled are left behind.
AEM 6.1, 6.2, 6.3
This is a known product issue GRANITE-18121 which is fixed in AEM 6.4.
If you are on AEM6.2, then install the attached package cq-6.2.0-patch-GRANITE-18121.zip. This would install a configuration for a scheduled task that cleans up the DROPPED jobs.
If you are on AEM6.3, then contact AEM Customer Care for 6.3 version of the package.