The timing of this is uncanny, we were just having a discussion about this internally. I've found that a great technique to dealing with difficult customers is

The timing of this is uncanny, we were just having a discussion about this internally. I've found that a great technique to dealing with difficult customers is to show them that you don't mind them taking their business elsewhere, as the OP mentions. It always amazes me at how quickly this disarms people and completely changes their attitude and approach. And another example of toxic customers: people paying with Groupons.