We developed a very effective strategy for this at a furniture store I used to work at; the moment the customer makes any suggestion of legal action, all our employees were trained to immediately say "I understand. Have a good day" and end the conversation on the spot. The unhappy customer immediately tries to press the issue, because what they want is for us to magically teleport a couch here from China or whatever, and at that point the employee says "I'm sorry but as you've notified us that this issue is now the subject of a pending legal action any further communication will have to go though our legal team."

Repeating this a couple more times would inevitably lead to the customer admitting that they were bluffing.