Hi!
Well, I am not sure what your question is regarding this topic. Do you need help organizing your ideas? Do you need proof-reading your finished essay? It appears that you have already chosen your topic, and that is the first difficult challenge. Start your letter without any "positive" build-up. This is not the sort of letter for that. Distance yourself, emotionally, from the employee and address the issue immediately. Then remind the employee why his/her conduct is inappropriate, and lastly explain the ramifications if his behavior is not corrected.