The Blank Slate: When is it Time to Fire a Client?

I've been working as a designer for a long time, and I've rarely had to let a client go. But there have been times when it was necessary. It can be a difficult task, first coming to the decision, and then communicating that decision to the client. But of course, there are always ways around confrontation.  After all, designers are also creative thinkers, right? So this brings me to the question...

When is it Time to Fire a Client?

I would like to know what brought you to the decision to let a client go, and did you let them down easy, or give it to them straight. Or maybe you found another way to get rid of what fast became a thorn in your side. Share your experiences below, and let the community know how you handle this type of situation.