Is it possible to change gain on Adafruit PN532
If a permanent change is OK, you can modify the antenna matching circuit to 
reduce the gain and thus the range. Here's the relevant part of the schematic:



The simplest way is to increase R3 and R4 to about 2-3 Ohm, which will reduce the Q-factor. Or, if you have the patience and the tools, you can re-implement the entire circuit according to this application note with the correct Q-factor and lower gain.