I have a 91 year old friend. He keeps hitting the ringer button on his Samsung phone and turning it off. Is there a way to lock it to keep this from happening?
Stock I can't think of any way to do this however there are volume controlling apps on the Play Store that would likely be able to accomplish this task. I personally cannot recommend one, perhaps one of our other members will see this and chime in with their favorite but my best advise would be to search volume at the Play Store. You should find several apps and from there you may need to do some trial and error testing to find the exact right fit.
In your reply please include which device it is as each device has different features and knowing what device we're talking about can improve the odds of finding the answer.