Don't worry about those that act like they know everything, and act like they are completely into Tolkien just because they have seen the films. Most of them are not truly fans, and are just into it because of the hype. Don't fret, they will be gone and not into it soon
And just to add... I am a person who read the books after the films, but actually ended up loving the books much, much more because of the detail and the style that they are written in. I would have never came here, or into this world, if the films had not come my way, and you could actually say that now I am forever grateful for them.
I don't think at all that the films have ruined the works of Tolkien. But then I also don't think that he would have liked all the hype that is around it now. He was a very quiet man who would have most definitely disliked it completely.
But remember that the books and films are really still two separate entities. If you dislike the films, the books are still there unchanged. The films are, and always will be, just an adaptation.