How to Restrict Calls to/from Phone

Hi! My grandfather has Alzheimer's and is notorious for answering every single phone call - including spam calls. He is not aware that they are spam calls, so I put a DND setting on that only enabled notifications from a handful of numbers. However, he has discovered that he can turn on and off the DND setting. He is also able to see his missed calls from the phone app and has started calling back the spam callers. Any ideas on how I can disable calls to and from his phone for an exception of the 5 numbers.