dude it's totes normal to get crushes in an instant, trust me i've had loads. If you know for certain that it will never happen, just don't bother yourself about him. If its not that easy then try putting yourself off of him, thats what I did-for example if I saw him teasing someone in a banter sort of way, in my head I'd make it seem as if he was being a bully. I know it's hard to turn off these emotions, but you've just got to tell yourself 'No, im not doing this again, i'm not getting my hopes up for nothing'