Technically yes, Let's Plays have made me NOT buy a video game, but it has made me BUY way more than not. I actually did a poll similar to this a few months ago, asking how many games people bought based on a Let's Play, ranging from 1-5 games to 20+. And the majority of people who voted, were in the 5+ categories. The smallest body of voters (as I can recall), were those who didn't use Let's Plays to buy games.
Personally, the only reason I didn't buy the games on the Let's Plays that I saw, was that the game just didn't appeal to me. It looked interesting at first, but after actually seeing how the game played, I realized it wasn't for me.
However, I've probably bought, at this point, about 20+ games, specifically because of Let's Plays. And maybe 2-3 were not bought by me, because of Let's Plays.