What can I use to block high frequency microwave radiation?

There is a shielding process used for this purpose . Electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of CONDUCTIVE or magnetic materials. read more

Microwave energy radiates at wavelengths ranging from 1 millimeter to 1 meter, and frequencies ranging from 300 GHz to 300 MHz. High frequency radio frequency energy is radiated at wavelengths from 10 meters to 100 meters, and frequencies from 30 MHz to 3 MHz. read more

If the frequency is not too high, you can get away with all-metal screening such as fine mesh copper screening or aluminum screening. Solid and continuous metal foil (thicker the better) will block very high frequency RF waves better than screening, but I would still use screening if I had no other choice. read more

Exactly, it's to protect you from the radiation. This is what is called a Faraday cage. Finally, if you are worried the microwaves from your microwave oven might harm you or your family, don't worry, you are perfectly safe, your microwave oven is essentially a microwave-oven-Faraday-cage. read more