My approach, when I find an Atcherley shoe-horned into the wrong family tree on Ancestry, is to place a comment on the profile page for that person. Direct messages to the tree owner are usually ignored, but by placing a comment which other users can see when they visit the page where the misplaced Atcherley appears, they have a chance to see why I believe the Ancestry user responsible is 'barking up the wrong family tree'. Sometimes the tree owner responds, mostly not (in many cases I'm guessing the tree owners are no longer Ancestry members), but almost always my comments remain online. I explain why I believe the wrong conclusion has been reached, and in some cases I have done a little research to identify where the tree owner has gone wrong (usually several generations before the misplaced Atcherley turns up!) and offer information which could set them off on the right track.
I find that most 'ancestor thieves' (people grabbing the wrong ancestor for their trees) are beginners. As others have pointed out, newcomers to genealogy can be too inexperienced to know that hints are just hints, or that not all events were recorded, that not all records have survived, and that that not all surviving records are online (you know the approach: that Elizabeth must be the Elizabeth who married John So-and-so as it's the only marriage of a John So-and-so and Elizabeth I've found on FamilySearch). In some cases no doubt they are too lazy or have too little time to put in the required research. Those Ancestry 'shaky leaves' sure can lead to some very shaky family trees!
Then there are the 'name collectors' who simply don't know when to stop and seem to judge the value of a family tree by the number of people in it rather than its accuracy or the extent of the information it holds telling you who all those people were and how they lived their lives. I tend not to mind name collectors so much nowadays, as long as the names they have collected from my trees are linked with the correct relatives.
It does worry me that so many people blindly copy whole chunks of other people's trees without checking the validity of the data they've grabbed, since others on seeing the same data repeated in several Ancestry trees may assume that it must be right purely because so many people say so. My answer to that, in the case of my Atcherley family tree, is my website where I have made accessible the tree according to my research, and where I will over time add details of the sources I have used to reach my genealogical conclusions, along with the stories of the people who appear in the tree.
Steve