We had an awful web
design client who we did lots of work for at really low rates because he kept
complaining about it and demanding free revisions.

I then sent him a bill for $1,200. He said he would pay it within a week. 

Seven days later the
bill was not paid. I sent him a message asking him about it. No reply, so I
sent another nice one. I called him, he said he will pay that night. He didn’t.
Two days later I called him again:

Client: Stop pressuring
me! I will pay it when I have time!

Me: Well, if time
is the issue you can just give me your credit card number and I will process
the payment for you.

This
person is not our client anymore