Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of crush],
While I am tremendously flattered by your interested in me, as I said before, I am not interested in pursuing a romantic relationship with you. I have become uncomfortable with the level of attention you have been giving me, and am asking you to cease. Please stop calling me or contacting.
I have said no several times, and I am not going to have a change of heart. If you don't stop calling me or coming by to see me, I am going to have to take more stringent action against you.
Thank you for respecting my wishes and stopping your attention.