It depends on the kind of existing relationship you have with this person. If it's someone you don't associate with everyday, try and steer clear of them as much as you can. If it is, try to point out their unattractive qualities - this is assuming it's more than just a physical crush. Worst comes to worst... time. I had a crush on a friend in high school. He ended up going to the U of Texas, I went to the U of Arizona. It all worked out.