Depends on your actual domain model.
Option 1: Change the 'User management' setting of the MendixAdmin role, such that the MendixAdmin user can manage the Roles of the Account you want to delete.
Option 2; Change the access rules for the applicable user role of MendixAdmin and enable 'Allow deleting existing objects'.
Option 3: If your Account has an association with delete behaviour, check that MendixAdmin also has permissions to delete the associated objects.