Bleach will take it right out. I carry a 1 gal sprayer with a 50/50 mix of bleach and water. I used it today, as a matter of fact, for some crepe myrtle stains.
I have had the same problem...Haven't tried bleach, but also haven't found anything else that will touch those stains...hot water does nothing. I'll try bleach the next time I run into them...Lots of crepe myrtles around here.