The sysadmin user can not be deleted. Think of that user as the superuser of the system (i.e. root on a linux system) that has full rights to do everything.
You may want to review the Managing Directory Services section of the Administering_My_webMethods_Server documentation that describes the purpose of the automatically defined users in the system directory service.
#webMethods#MWS-CAF-Task-Engine#webMethods-BPMS