Under the file menu click Manage, Database. Click the Relationship tab. Then select the table occurance related to your portal. Edit the relationship and uncheck allow creation of records in this table via this relaionship.
You may need to create another table occurance if you will need to create records from the original portal. You would have one table occurance with allow creation checked and another occurance with it unchecked. Then you would use the table occurance with it unchecked in the portal than you do not want the blank field(s) at the bottom of the portal.