A polite way to tell someone to fuck off and die because that person has created the circumstance between you and them which makes you care less if you ever see them again.
He could not deal with her bullshit and destructiveness in their relationship, so he signed the letter "have a nice life."
In a world of so much bad and complete sentences, having a nice is an open ended term and philosophical way of living life of looking at the simple things and being grateful for existence as it is.
A very cold - but humorous - Hollywood dismissal derived from the commonplace parting “have a nice day,” but taken to the utmost extreme. “Have a nice century” is even more insulting than telling somebody to “have a nice life”. “Have a nice century” implies to the person receiving the dismissal, “you are literally of zero worth to me and always will be – I’m completely sure that your life on this Earth will end with such little significance, that I will never have to concern myself with your petty existence again,” or, in short, “you’re done.” This phrase was originally coined by the music artist Solo (Rowland), and since has become common around Hollywood – especially by talent agents, managers, directors & movie producers like Mike Lewis, the Vice President of Crown Model Management.
As the arrogant would-be model marched out of his office shouting profanities, Mr. Lewis calmly responded with one line: "have a nice century". At that moment, she knew: her hopes of ever making it big in the entertainment industry were gone.
It basically means goodbye forever. If you say this to another individual, you are telling them, from now on, from this moment on, you couldn't care less what happens to them, as long as you never see, or hear from them again. You are banishing them from your circle of loved ones and friends, from your life. It is almost always justified/warranted.
"Don't worry about responding to this. I just wanted you to know how I felt. Have a nice life."