You're right, this will be a fast way to get people to stop telling you their ideas. But if you're trying to shut people up then you've missed the OP's point.
You can easily make normal people feel like their idea is worthless and you have mountains of business acumen that, obviously, they must lack. Do this enough times and what you'll find, just like you'd hoped, people stop telling you their ideas.
I'd argue that every time you do this you are shutting down opportunities to collaboratively create ideas that would probably work. Part of being "lucky" is increasing your exposure to failures [1].
When someone approaches you with an idea, you've certainly got a better feel for what will "work" more than you're average person. Consider taking the time to give feedback, help shape it, brainstorm with it. You might find that between the two of you what you create is bigger than what either one could come up with individually.
This applies in all areas of personal relationships, not just iPhone games.
[1] "To increase your chance of success, double your failure rate" - attributed to Tom Watson