Mom with dementia using our microwave. Any advice?

The stove is usually on its own circuit and can easily be turned off from your electrical panel (if the panel is easy access). Sometimes the microwave or the fridge are on separate circuits and you can turn the microwave off the same way. If the fridge and microwave are on the same circuit, you might want to consider adding a power strip (usually with surge protector) so you can easier switch the microwave on/off.

You might want to try teaching your mom to use time settings on the microwave. The start button on my microwave runs the microwave for 30 seconds with each push. It took a few times, but eventually got her to just use "start" to reheat because "that's so much easier".

Or just unplug the microwave and take the knobs off of the stove when you're away. Sounds like mom needs more care though and should not be left alone.

When Dad lived with me, I didn't allow him to use either. But I was able to be home with him 24/7 unless I had someone to relieve me. And his mobility was so very slow that I could check to see what he was doing before he could leave his room.