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.
"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 ......"