What is the best way to tell a professor that my research interests are not limited to a specific topic?
I guess the answer depends on what kind of information you want to convey, and for what reason you want to make it clear that your interests are not limited to X area. "My research interests include X" is already non-exclusive - just because you say that you are interested in X doesn't mean that you aren't also interested in Y and Z, and most professors realize that students (and all their colleagues) have more than one small narrow area of interest. So you can just say that and be fine.

Basically, my point is that it's absolutely fine to tell a professor that you are interested in one specific area, as long as that area is sufficiently broad in your field. The goal is to find a professor whose interests match or are complementary to yours, so when you say "I am interested in X" he thinks "Ah, I have always wanted a student who would study X!" or at least "Hmmm, it might be good to have a student in my lab/group who studies X."

However, I would not say "My main research area is X, but I am open to other subjects as well." That makes it sound like you would be willing to change your area of interest if the right PI came along or to get into a specific program, and you don't want to convey that, as counterintuitive as that may seem. It makes you sound wishy-washy. You want to be regarded as a serious scholar with a deep interest in a particular area.