How to fend off someone putting their Name on my work?
Your situation sounds messy. I think most will agree that the basic prerequisite for co-authorship is intellectual contribution to the manuscript . To add authors for thanks is common but really not a practice to endorse. If someone comes to me and asks to be a co-author, the obvious response would be: "What will be your contribution?" If there is no clear contribution other than, say, "Read the manuscript and provide input" then the answer should be "No, thanks". In your case, it sounds as if there is no major point in even having a discussion.

In fact, your case reminds me of a colleague of mine who ends up in all sorts of strange situations. His problem is that he is too nice and does not know the word "no". I realize it may be hard to set up strict rules for these things if your immediate surroundings do not subscribe to them, but I think you should know that you have solid grounds for being more strict about adding unnecessary co-authors or letting people into a project without a clear picture of why.

If you are first author on a paper, it seems to me you will also be the person deciding how the work should materialize. I think you need to make your own mind up where you want to draw the line. you will benefit of making a decision on how you want things run. If you want to deviate from this later on, then it is still your decision and no-one else's. I think this is what you (and most of us) really want - to be in the driver's seat. So follow your instincts, they seem sound to me.