Not sure there’s a group option for this.
Try to disable the module in ACP -> System -> Applications-> click in System to expand -> Messenger-> padlock -> uncheck the group.
That will make theses users to not use it but I’m not sure will work.
Test is simple: buy a gift card for someone else, choose PM and the other user will receive the message. That’s all.
It worked on last update, unless something has changed… so make a test and let me know.
Just added the Secret Key and it's working just fine:
Will release a new version.
You can add your Secret phrase in the screenshotmachine site by editing your account:
This was tested on localhost, fresh install with no 3rd-party resources.
Although I select the option to anonymize the content, it keeps attributing it to the deleted member.