You're trying to take away communications capabilities from a tool designed to communicate. It's probably not the best choice of devices.
You can start by setting the phone into "airplane mode", which is intended to shut off the radios. Because of the way RF works, that means it shuts off both transmitting and receiving. It should keep you safe, but of course, you could accidentally enable it again.
You need to take a survey of the RF that the phone is designed to communicate on. Obviously it has cell communications, but it may also have Wi-Fi, Bluetooth, and NFC. If you disable the cell antenna you won't stop the other communications channels. You'll have to also remove the hardware that supports the other technologies, and do so without damaging the phone. (For example, a Bluetooth or Wi-Fi antenna is a very tiny element on the circuit board.)