Play it back to them. It works!

I found something that works. 100% success rate the several times I’ve done this. You need to be able to be ok with a little confrontation but it works. 

Record a part of the speakerphone call, maybe 30 seconds. Then play it back on your speaker over and over and suddenly YOU are the crazy person and they quickly understand how privacy is important. I’ve gotten the same exact dirty looks we’re used to giving speakerphone users.

[How the turntables have turned meme]

Making them have to hear their own voice they become self conscious and it’s annoying having to speak over themselves so they stop.