When learning how to handle customer complaints effectively, you have to be prepared for the customer typing in all caps.
Dealing with these customers, especially on live chat, is an art. Some times customers have a problem and need to blow off some steam, and they're going to blow that steam off in the face of the first person they can reach on live chat.
Is it fun? No. Does it happen? Yes, some times. Can they be reasoned with? Absolutely.
An Olark user recently asked me the following: "If you had a customer who was talking to you in all caps and that customer clearly isn't mad, would you ask him to politely turn off caps?"
My response? "No. I would continue the conversation and not call attention to the fact that the customer was using all caps."