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Let him know that you are not interested; say this assertively but without being mean. You do not need to list all of his faults and hurt his feelings. Just be clear that you don't think a relationship (of whatever sort is at stake) will work and that you would prefer it if he leave you alone.  For example, if he keeps trying to get you to go on a date and you want him to stop, you might say "look, I'm sorry but I am not interested in dating you, can you please stop asking?" If the honest reasons may actually hurt the individual (such as if you find him annoying), re-phrase the reason to make it less hurtful. For example, if he asks why you will not date him, instead of saying something along the lines of “I find you annoying” you could say, “our personalities clash and I do not think we get along that well”. This removes the emphasis from his character and places it on the particular dynamic that occurs between the two of you. Empathy leads people to act more prosocially. Let him know that the way he is behaving towards you is making you uncomfortable or scared, and that the experience is unsettling. He may not realize that his attention toward you is making you feel uncomfortable; he may reach the opposite conclusion, that you like his affection and attention. Engage his emotions by telling him how you really feel about his unwanted behavior.  For example, if he still is still pursuing you even though you have said your personalities do not match, you might say "I have told you that I am not interested a number of times and I feel like you aren't listening, which is making me feel uncomfortable and nervous." Do not give him any wiggle room to misinterpret your words. If you do so, he may stick around or give you distance only temporarily. Rather than saying “I’m not interested in dating you right now” close the door entirely by saying “I am not interested in dating you”. In severe cases of unwanted attention, after all other options have failed and you truly feel unsafe, threaten legal action. Be ready to turn him over to the police if he continues. Let him know that you are taking detailed notes about his actions toward you. Keep a record of all of the communications attempts he makes.
Be honest. Encourage his empathy. Don’t leave any doors open. Threaten legal action.