You've got something or several somethings not set up correctly.
Here's what you should have to get this to work:
Your portal should be based on the Join table, tblSalesJoin--not either "many" table. You can include fields from tblExtras in this portal to show data from that table in the portal.
In the relationship TblOpportunity---<TblSalesJoin, you can enable the delete option for TblSalesJoin records and you can enable the delete option for TbleSalesJoin records in the relationship: tblExtras >-----TblSalesJoin to enforce referential integrity. Do not select the delete option for either tblOpportunity nor tblExtras.
Now deleting a record from the portal will delete the Join record, will delete the Join record but leave the related records unaffected.