My 14 month old has been doing this continually and it has been causing major problems as older ds is watching TV and gets really annoyed. We have tried saying "no" and distracting him and also tried 1 minute time out in his cot each time he does it. None of this works. And although totally ignorning him would probably help, this is not possible as ds1 reacts majorly when ds2 turns tv off.
Anyway, finally found a solution today! We have a very large screen (extremely heavy) tv that sits in a wooden tv cabinet. The button to the tv is at the bottom of the TV. We took two wall brackets (the metal ones that are at a right angle)and taped them together with electrical tape. Then we put one end of the bracket under the tv (took two of us to lift it!). The other end then comes up and sits in front of the button. This stops ds from reaching it. So, how do we turn it on? Well, the brackets have holes in where the screws go. We are using the end of an allen key to poke through the hole and press the button when needed!
SOunds insane that we have gone to all this trouble, but anyone who has a toddler turning their tv on and off will understand! I hope this helps one of them too