Well, what you would really like to do is make the users actually read the policy agreement before clicking Yes/No, and, of course, that is impossible. Forcing them to scroll is not going to force them to read, but I expect you know that.
If you really want this (rather than, say, spending your time editing the policy statement to be shorter, and plainer English) then you are going to have to write some custom JavaScript that
- Disables the button on page-load.
- Re-enables in once the user has scrolled to the bottom.