Restrict Grid only to Add New

Hello Experts,

On a specific requirement I am adding an opportunity subgrid to the appointment form which will enable user to "create" a new opportunity derived from that meeting. I want to restrict the grid in such a way that it will ONY ALLOW to add a NEW OPPORTUNITY. Right now when I click on the + button to add it is allowing to select an existing opportunity and/or create a new opportunity. How can I restrict the subgrid to allow only NEW opportunity.

Thanks

AbD