Dunno. Like I said, there's no way to do this effectively without some form of leverage over the counter-party. This sort of thing is why SCIF's exist, and is

Dunno. Like I said, there's no way to do this effectively without some form of leverage over the counter-party. This sort of thing is why SCIF's exist, and is an example of the more extreme ends of leverage, but it still ultimately comes down to leverage: they can make you delete the message and will throw you in jail if you figure out a way to evade it. One-time secret, maybe?