Dear Mary: How can I spike a gossip-pedlar’s guns?

Q. On arrival at a top level dinner, I was surprised to see at the table a woman who, I have reason to suspect, sells gossip as a sideline. However, clearly no one else suspected her and, assuming it was Chatham House rules, everyone was talking freely. When one man began to regale the table with an anecdote which was bound to culminate in a dynamite piece of gossip, I was paralysed with horror but I couldn’t think how to stop him before it was too late.