Try posting a comment to the photo as it appears on this tree, something like: "FYI, this is not John Jones b. 1900 and d. 1945; it is my great grandfather (also named John Jones), b. 1890 and d. 1952, and this photo is from my family's personal collection."

Now - the person can just delete your comment, and there won't be much you can do about that, but now that you are alerted you can follow any copy that is made, and give the same information to those who perpetuate the error. About the only way to "break the connection," however, is to get it off that tree and that's not something you will likely be able to force, I'm sorry to say.