Hi, it happens all of the time I'm afraid, it's one of the pitfalls of using Ancestry if you have a public tree, the only alternative is to make your tree "Private", sad I know, but it's a harsh fact of life.
If it was me with the problem I would "Have a pop" at the cousin

I'm pretty good at pointed remarks, something along the lines of "Hey, are you going to share some of your research with me, or are you going to use my time and money all of the time?"
Just to see what reaction I would get.
Frank.