This little script removes images which appear to be in the portrait format (tweak it for your own needs).

#!/bin/bash

orientation=`exiftool -orientation "$1"`

echo "$1" has $orientation;

if ! $orientation =~ "Horizontal" then

   echo "so removed";
   rm "$1";

fi

You can run it recursively with: find directory/ -name "*.JPG" -exec ./rm-portrait {} \;