This would be hard to achieve i suppose. You can try installing the Packet Filter plugin and setting the blocking rules rules for message and presence packets. But i’m afraid this will disable chats in group chat too. If not, there is still an option to have a private chat inside the group chat. So, i think you still will need a custom solution for this (in a form of a plugin). Can’t suggest how to build one. You will need a java developer who will dive into Openfire code and figure this out and write one for you.