you may construct an active or passive IR Barrier with an 89C51.

fisrt the active one:
togge a port pin with approx 36kHz, connect an IR Diode to it - e voila an active IR Barrier that disrupts all IR Transmission in range (but you probably want a NE555 for that job)

the passive one is much simpler and requires almost no MCS-51 knowledge.

take a PLCC 89C51 and glue it in front of the IR transmitter.

Cheers, Roger