How about subterfuge?

You could line up a conspirator, who is in on the game and won't take a rebuke personally, and have him/her reach for something in the other person's presence. Then you can say something direct and clear, with no worry about offending anyone's delicate sensibilities. And maybe this guy will hear and get it indirectly...

Or if that fails, maybe you could put some tantalizing (but not valuable) object in reach that is booby-trapped somehow, like maybe it imparts a (mild) electric shock?