Here are the things that I find that works to stop even the toughest customer that wants the tree topped no matter what.
1. Topping it will create a more frequent trim schedule and cost you more in the end. Topping that tree will put that tree in stress mode. In no time (2 to 3 years) it will be as tall as it is now, if not taller, but thicker and weaker than ever before.
Topping it will create a frequent maintenance interval. You are going to want it trimmed every 3 or 4 years if you have it topped.
If you let me properly trim it with a mild crown reduction, thinning and dead wooding, the tree won't "notice" that it was trimmed and it will stay in the new relative shape MUCH longer. You will likely get 7 years out of it before it might need a trimming again.
The more drastic the trimming, the worse the tree will react.
2. I'm an ISA Certified Arborist and a MD Licensed Tree Expert. I can't be seen doing improper trimming. We only do trimming that will keep the trees healthy and safe. I can't do something that will likely cause an unsafe or unhealthy tree and that's what topping does; create an unsafe or unhealthy tree.
3. And I often give this analogy of mine. You can use it if you like it. It really seems to work. Just remember you got it from the Xman:
I can't top that tree, topping is wrong.
It's like this, let’s say you went to a medical doctor and said, "Doctor, I don't like how long my fingernails are, I want you to cut my fingers off half way".
A good Doctor would explain; NO, do NOT DO THAT! There is a proper location of where you should trim the fingernails. Cutting the fingers off half way would be very unhealthy and ridiculous. Please, let me just trim the fingernails at the proper location.
A bad Doctor would say, sure, you pay me and I'll cut them fingers off anywhere you want me to. If that doctor did this, they should be sued for malpractice.
I am the Certified Arborist; I am like the good doctor. I am telling you how your tree should be trimmed. I refuse to be the bad doctor.