Keep outer joined table from saving record in form

Hello,

I have a form that has the primary datasource as InventTable and CustVendExternalItem table joined as a outer join. The CustVendExternalItem datasource is set to not allow creation, editing I make a change to the InventTable, it tries to save to the CustVendExternalItem table as well. I know this is probably something simple, but I don’t know what it is. How can I have CustVendExternalItem joined to InventTable to where I can save changes to InventTable without changing CustVendExternalItem?

Shawn