You don't do that sort of notice until after someone did something dumb, and you have to make sure the notice you put up is slightly obscure and not really helpful. Something like "microwaves are provided for heating up food items only. Thanks!!!!"

Then people can get upset about how the sign is really obvious so it's treating them like they're stupid, then someone else can get upset because it says they can't microwave their wheat bag and that's discrimination because they use it for period cramps. After lots of outrage someone will decide you can eat wheat so technically it's food so heating it is ok. Eventually after everyone is ignoring it anyway the sign will fall off and no one will notice.

At some point after that someone will try to dry their umbrella in the microwave and it will catch on fire, and when they get told off they will claim they had no way of knowing they shouldn't because there was no sign saying not to.