Customizing ribbon on specific entity form

I have an entity (Team Member) that is a collection of contacts (with other data) that defines a team of contacts from the client.  The Team Member entity should be only and add or delete since a single Team Member record can only be related to ONE client (Parent relationship).  When I display the Team Members grid form, in the ribbon there is a button to "Add Existing".  Since this isn't possible, if the user tries it throws an error (as it should).

My question is this:  How do I customize the ribbon on this particular entity/form to remove the "Add Existing" button.  OR, is there some other solution OR am I doing something wrong in my definition of the entity relationship?

Thanks,
Glen