I just had an idea for those every other week through the season customers (and possibly others). What about explaining to them the proper height of cut and the one third rule and all the negatives of violating it. Then come up with a measuring stake (red mark at cut area) to be placed in an out of the way place for them to see exactly when it needs to be cut. Sort of like the water level markers in lakes. I wouldn't leave it forever, just long enough for them to get the message.