I found this in Karl Albrecht's book Practical Intelligence, which I thought was going to be more about approaching problems logically but was filled with a lot of wishy-washy advice.
This is his suggested speech for "firing" a toxic person from your life:
"I've been thinking about my life lately, and I've been deciding what my personal priorities really are. I've decided that I only want positive relationships with positive people. I don't know how to build a positive relationship with you that serves my needs, so I've decided not to see you any more. I don't harbour any animosity toward you; I just find that there's no place in my life for this relationship any longer."
Not to be too blunt, but what a load of verbiose garbage.
"It's not you, it's me" is a heck of a lot shorter and to the point.
Looking at it, it strikes me as pompous and arrogant and designed to prevent the other person from expressing themselves. If you're "firing" someone then the least you can do is let them get their final say in. You're walking away after this, so this is their last chance to ask questions, offer apologies or indulge in begging.
If I thought someone was mean enough to be really harsh, then a break-up speech is a bad idea. Just cut them off. Eventually they'll get the clue. This kind of speech is something you'd want to use only if you still care about the other person's feelings and want them to walk away reasonably satisfied. It's supposed to set up closure.
I prefer Sandra Bullock's line from Divine Secrets:
I'm done. That's gone with a D.