If I feel that someone is crushing on me, but I'm not interested in them, I very subtly discourage them from pursuing me. I don't think I could just say "Nope, sorry, not interested" in such a blasé manner and feel OK about having so behaved. In uni, I knew one of my friends was slightly crushing on me, but I wasn't interested. So one day I conspired to bring about a conversation on relationships and partners. E.g. I told her that I couldn't ever have a relationship with someone who practices a religion (she practises her religion), and that I'm very open and specific about what I want from a relationship/partner.
I asked her about her perfect man, and repeatedly (in a jokey manner) pointed out that I was not the person she sought. Thankfully, she picked up that oft in words spoken in jest you will discover the truth. We're still good friends, and she doesn't crush on me, because I'm demonstrably not for her, nor she for me, as anything more than the friends we are.
Has happened a few times. I start to put my attention more on other friends or just start to ignore a little bit. Usually shakes off any attempts at confessions. Then I slowly inch back to how often we would use to interact.
If I notice one of my friends has a crush on me, I try to drop hints that I only see them as friends and that they should move on. It's too awkward to face head on, especially of they haven't confessed.