From my experience I find that negative advertising doesn't work that well.
Sometimes even if the customer bought a lemon they'll stand by it because to do otherwise may make them look the fool.
So you can hear things like "oh yes, they were marvelous, real pro's, they had those things on their feet and went all the way up there and did it, they really knew what they were doing." Now it's a long road to turn that around, they may dig in and defend the culprit, and when they finally realize the damage done they might not call you because they'll have egg on their face.
Try to go in on the positive approach, Ask questions, I'm a qualified arborist and noticed that you recently had some tree work done, are you happy with the result? If they say yes, "well that's great, I also do a professional job and work to ANSI xyz standards, so if you would like to look after your trees, make them safe and beautiful please consider calling me, here's some information on correct pruning" (and leave some literature)... If the customer's response to the opening question was no, well,you can go to town.
I do convert quite a few people from lopping/topping at the quote stage, if you don't go you'll have no show. So, I wouldn't negative advertise. On my website I have a dedicated section to Stop Before You Lop and tell people to read it. I also have a Tips & Hints section that shows the correct cuts, so if I get a phone call for topping I ask them to read that before I go out, and when I get there most of the time convert them to a prune, thin or takedown.
