Put a button in the portal row and define it to use the delete portal row action. Make sure that you have not enabled the "delete" option for the parent table in any relationship linking the portal's table to the parents. Also enable "allow deletion of portal row" for the portal in portal set up.
You can also use a script for deleting the portal row that is a bit more user friendly. There's a script for deleting portal rows found in the Known Bugs List Database that you can import into your files. It can be used to delete portal rows from any portal on any layout in your file, but you have to read the comments in the script (and maybe check the button setups for the delete buttons in the portals in the database) to determine what script parameter expression to use to pass the needed info to this script.
Thanks for the help. I have another question.
Is there someway to filter portal records based on whether or not a record exists in an unrelated table?
Not if it's an unrelated table. There has to be some way to reference the correct record in the other table. But there are many ways to set up relationships, including using the X operator to match any record in table 1 to all records in table 2.