<p>As other posters have said, just tell them how hard it is to get in:</p>
<p>"Thanks for the encouragement! I'm not expecting an acceptance though, given that only 10 percent of the kids get in..." or something of the sort. You don't want to turn them off by being abrupt, but you also want them to be aware that it's not necessarily going to work out as they think. Better to make sure they're aware of that now than later, when they hear you got rejected and are really confused about it.</p>
<p>Also, I don't think you explained how anyone would know about your college list in the first place. I find it much easier just not to talk about it at all:</p>
<p>"Oh, I'm still looking. No school in particular." works until I apply. Then it will probably be "I would rather not talk about the applications, because nothing is certain. But I will definitely let you know where I decide to go!" is probably what I'll say after I send in my apps.</p>
<p>In order to avoid nosiness, just shut out all possibilities of further questions. If you say, "Oh, I'm not sure about all my schools yet" you open the possibility for the next question, "What are(/is) some of them/your top choice?"</p>