Is there a phone service that interrupts/deflects/scans a demented persons incessant phone calls?

Your profile says that the person lives with you. Is that still the case? If so, is she using a cell or a land line? In the case of the land line, unplug all the cords besides the power cord and keep them away from her. If we’re talking cell phone, put a screen lock on it and change the code frequently. If she’s that far gone, you need to have her evaluated.

In Canada you can block certain numbers, would that help? Is you LO in a facility where there are competent people who can call in case of a real emergency?

Otherwise call display and do not answer?

Disable the phone?

If they are calling your cell, set the number they call from to no ring tone.

I'm not aware of a service like that, but, if the person is that progressed, they would benefit from being under supervision. Whoever is supervising them can then direct the calls, call for them or call you if there is an emergency.