Yea we had this problem occasionly as we have a top level indicating what department it should go to and people would just pick the department, so we added a rule that rejects a ticket if it's entered on the top level.
But onto specifying helptopic based on url http://domain.com/open.php?topicId=2
Something like that should work, you can find the id of your help topics in the admin panel, hover over the link to the help topic and note the id in the link.