I need advice on how to communicate with the the gardeners I hire to mow and edge my front yard...

My yard has lots of large trees and shrubs, most of them slow growing. The lawn is fescue. I have large tall hedges and several shrubs that are clipped into shapes (cubes, domes, mushrooms, globes)

The problem I am having is the lawnmowers want to be too helpful and clip my shrubs and trees. This morning I discovered they had cut the main leader of a small gingko tree that I am (was) bonsai'ing. It made me very angry and has ruined my day. The tree will be in the trash this week - a waste of several years time.

When I hired them I made it clear all I wanted was a weekly mow and edge (I take care of pruning clipping myself). I've had similar and worse problems with past gardeners. The current gardeners have been working 3 weeks and have done a fine job mowing and edging.

Do I talk to them some more or fire them? Do I talk to just their "main guy" or all of them as a group? Do I put it in writing? Help!