Hilchos Lashon Hara, chapter 2, section 7

The leniency of speaking lashon hara that was spoken before three only applies if the speaker spoke with no strings attached. But if he warned them to not reveal the matter further, it is forbidden to speak of it further. Even if he sees that one or two of the listeners did speak of it further despite the warning, it is still forbidden to speak of it to other people.