How to switch on/off RFID using piezo sensor?
This cannot be achieved by modifying the antenna path. Instead you should probably detect the press and turn the entire circuit on/off.

If instead of a piezo element, you use capacitive touch then you can use off-the-shelf components and circuitry. For example dedicated capacitive touch IC like TPP223-BA6, or AVR AtTiny microcontroller with QTouch library.