Limit how high they can jump!
I have made this suggestion a few times and never heard a good reason why not ... limit each level of hog as to the level of wall they can jump so, for example,
lvl1 can jump walls level 1-5
lvl2 can jump 1-6
lvl3 can jump 1-7
lvl4 can jump 1-8 and
lvl5 can jump 1-9
That way SC encourages 'investment' in walls, re-engages the disenfranchised people who have already made the investment and now feel it was a complete waste of money/time, and all will be well in the world ;)