Restricting Filemaker Pro 10 users from deleting master records only related records

I have two relational Filemaker Pro 10 databases, master & slave. The master contains the general information about the item and the slave contains the sub components that make up the bill of materials for that item. In the master file, users enter the sub components in a portal of the slave file.  Is there a way to restrict a user from deleting the master record in the master file and only allow deletions of a related record in the slave file. I realize that I can select "Allow deletions of portal records" in portal setup, but it still allows the user to accidentally select the master record when the popup window says "Do you want to delete the entire master record or just this one related record? <master>  <related>  <cancel>". Certain users to need the ability add master records at the master level, but cannot delete master records and they can add or delete slave records in the slave file. Any thoughts would be appreciated.