How do I do this politely?

A month or two ago, a Jehovah's Witness came to my house. She was a really sweet elderly woman and I didn't have the heart to shut the door on her. I have to admit I do enjoy learning about and discussing different religions, so I took the literature she offered and was polite. I skimmed through it and did find it interesting, but all along I knew that it wasn't for me, because I am a baptized and practicing Lutheran and it's a really good fit for me. Anyway, long story short, she came back today! She gave me two more thin pamphlets and a small book this time. The next time she comes (I am uming there will be a next time), what's the most polite way for me to tell her...that she's sort of wasting her time? I have to admit I'm pretty dumb when it comes to this...like, is she out to convert me? I'd like to be polite to her and I don't mind reading about her beliefs, but it's just not for me. What's the best way to tell her this, and discourage her from coming back every month? :/