How does Near Field Communication protect from unwanted long-distance communication?
I don't really want to help some dirtbag steal other people's data, so this answer just discusses near field theory.

Far field is where the energy from the transmitter has organized itself into a propagating wave that keeps going on its own.  Both the E and B fields have to be envolved and interact in a specific way for this to happen.  The point of a antenna is to excite both the E and B fields in its vicinity the right way to maximize the portion of the energy dumped into the antenna that will eventually radiate outwards.

Since it requires both a E and B field to propagate RF energy, transmissions intended to stay in the near field use one or the other by try not to create both.  This means near field uses either capacitive coupling (E field) or transformer coupling (B field).  Since it is impossilbe to create a varying E field wihout some B field and vice versa(*), some of the energy will escape the near field and propagate outwards as RF.  This could be detected with a sensitive enough receiver if interference is low enough.

(*) Creating a varying E field takes changing voltage, which due to capacitance requires current, which creates a B field.  To create a varying B field requires changing current, which due to resistance requires voltage, which creates a E field.