How to “fire” a client

Question:
I know that a number of people have “fired” clients. I’m not sure how to fire a client, or at least decline a related matter from an existing client who is a real pain. I don’t want to have to deal with him on other matters. What excuses do you use to fire or decline work from existing clients? What should I do?

Response:
Here’s one contrarian thought:

My own preference is to enable clients and prospects to self-select. In other words, I don’t