Restrict IR sensor field of view
Almost with out exception COTS IR remote control receivers contain an AGC circuit.  The Vishay part used contains such a circuit (see block diagram at the bottom of page 1).  This feature allows for dependable detection regardless if the IR transmitter signal is received directly or if received bounced off a wall. 

What is needed in this application is a beam interruption detector.  These are less common but Vishay makes several including this one.  Note that there is no AGC circuit in the block diagram.  When the signal drops below a certain threshold the output goes inactive.