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Can you see their window?

Pick up a cheap CCTV infrared floodlight, gut it, and gate it with Ye Olde Bloody Great MOSFET driven by your TV-Be-Gone microcontroller.

Unless you manage to aim a strong IR blaster at their window/the ceiling behind it.

Indeed. It works with infrared light, the same way most TV remotes do.

A spark gap does pretty well. But the FCC fowns on such things.