You cannot directly delete the owner account in BQE CORE. You need to first transfer the ownership to another user before you can delete the original owner. Check CORE Help Center for details on that. After that, you need to de-select any packages assigned to the owner account in the Manage Users screen (Settings > Access & Permissions). Then you can delete that user or make it inactive using the Actions menu there. Check out this FAQ for more details.
CORE Help Center