How can I direct an RFID reader to only read in a specific direction?
You can place metal strips between the unwanted RFID tags (NoTags) and the reader that still leave a path to your tag of interest (YesTag). This will reduce the RFID reader field strength but it will work perfectly well if the distance from reader to YesTag is short enough and the shields are far back enough. These don't need to be grounded.

Your diagram doesn't show any distances though so I can't say either way. If you've got the set-up you've drawn, you can find out quick enough, obviously.

Last year I did quite a lot of experimenting with RFID tags and readers as we were mounting it amongst a lot of steel. Incidentally, our reader could detect and identify multiple tags within its field but that's probably not helping what you're trying to do.

(Incidentally, 'beam' is a harmless enough word, we all know perfectly well what you mean :-) )