When they ask for "Mr Sirius", simply tell them "I'm afraid he can't come to the phone right now, he's been sent down for rape/murder/manslaughter" (delete as applicable)
I find that has an 'interesting' effect, and right enough, then don't ring back
Or maybe you could get a sound f/x tape playing in the background, something like Texas Chain Saw Massacre, and just start whimpering, they'll get the hint.
