I want to remove the "Add ..." Button from the CMS display for a page that belongs to a many-many relationship. I only want someone to be able to click the checkbox to associate the Model object, but not create a new Model object from this page.
You can set the permissions of a CTF one of those being add. See http://doc.silverstripe.org/tablelistfield#permissions for an example of how to set it. In your case you probably want only view, maybe edit.