In layout mode double click the portal and check the 'Allow deletion of portal records'. With this option you can now build a script that will delete just the highlighted portal record or a specific one based on a calculation.
"Deletion of portal row without having to delete related record"
Well, the portal IS the related record. If you mean that you want to delete the portal record but not the parent record you are currently on then listen to Martin and that is probably what you mean.
Or do you mean that, when you delete a portal record that it deletes a record in a different table - one that is related to your portal table occurrence? If a 'grand-child' record is being deleted when you delete a portal record then it would point to 'allow deletion' being checked in the relationship between your portal table occurrence and that grand-child table occurrence.