Technology question: Any devices to prevent television from turning on?

Yes, those lamp timers can be set to go off at a certain time and come on at a certain time. Not sure though if it keeps the item from being turned on in between. For now try the timer on the TV.

What about simply plugging the TV into a timer?

As SusanA43 said most tv's have programmes in their menus. A lot have a 'child lock' option too. This might work, as long as you make sure you remember the lock number. :)

Alternative wacky idea from me. If mum wakes up and seems to need to watch something, how about a Lava lamp.

When some of our grand children stayed over (when they were younger) they were indoctrinated, it seems, in always having a TV in their bedrooms. We did not have one in that room. SOOOO, we put a small shelf in one of the corners, as close to the ceiling as we could, put a Lava Lamp - safely on it. BOOm, they had something to watch. luckily it was also relaxing, so double bonus, it helped them sleep.

Good luck

There are timers that will turn the TV OFF at a certain time, and most newer TVs have them built into their programming - but preventing it from turning ON is another matter. I know of no way to do that other than hiding the remote or unplugging the TV - then I'm pretty sure Mom would be waking you up to find out why she can't turn it on.

Is Mom hard of hearing? Is getting her a hearing aid - or at least some wireless headphones to wear while she watches TV at night a solution? There are wireless devices with headphones that will allow her to watch at night and be able to hear it through the headphones without disturbing you.

Don't know of such a device, but I've gotten used to sleeping with ear plugs