how to close/purge subcases

Hi, All

   I've encoutnered an issue about subcases. These subcases (same subprocedure) have been started some other parent procedures. Although they are subcase, they will also goes to the group/user queue and processed just like normal workitem. Now when I purging all cases in the system and suddenly found this issue, is there a way I can close/purge them? They do existed in staffo, caseInformation tables. 

   About how to fix this permanantely, maybe need check the design, this subprocedure are used for 'Notification' and are created by other parent procedures. It just like simple alert message, but it's behaviour is like a normal workitem. I'm not sure whether our design has problem.