You don't. There is no way that message can make you feel good about yourself, no matter how you wordsmith it. It's one thing to dismiss someone if you want to end a relationship, but saying things that still impact you negatively is not conducive to how you want to, or should want to feel. It is better to forgive them and if need be, dismiss them.
Don't directly tell them. Cut off all communications with them. They will soon learn that they can't contact you by phone or email and you don't have to have any face to face confrontations with them.
There's a song which states: 'you say it best when you say nothing at all.' I had stopped all contact with my family in Jan. 95. I guess at some point they got the message. I'm better off without them.
I hate you and please go eat some **** and die because I do not give a flying **** about you. Be sure to send me a post card from what was it, oh yeah hell.
"Dear sir/madame. I hate you and wish to disown you. Thank you for your time and I wish you the best for the rest of your life. Yours sincerely ......"