I mean theoretically if you are hosting your own chat server, for example on Matrix, you can easily make all the chats unaccessible from the clients by issuing a command to shutdown your server or simply the chat server service if there's no content cached locally.

I think you can do this pretty easily with a raspberry pi by connecting via ssh..

Just use a shell script that changes the static ip to something else after the command to shutdown the service/wipe out the data (depending on what your goal is) has been issued, or use a vpn or something like that if possible, because anyone issuing the command would need to know your server ip.

And issuing a command by ssh to a remote server both from smartphone or pc should be as easy that you can actually build a very small app for that, or use some app that creates shortcuts that directly connects and issue custom commands.

That way you are forced to give people your new ip every time chats become unaccessible/deleted and someone can't connect back even if wanting to without talking to you, unless you decide you can use the older ip for whatever reason.

Of course not using your real ip but using some service like a vpn or proxy (or tor?) would be much better here, but i don't really know how.

That can give you full power on the chat history and create the said "panic button" for every client involved.