Easiest thing would be to put sharpie on the LEDs. Second to that, pop off the SOT23 NFET just off the corner of the microcontroller, nexto the LEDs.

Or if you got a PIC programmer, I can conjure up a version of the firmware that doesn't flash the LED or flashes it only briefly so it's not as intense.

Or, you can tell your neighbor to stick it or hang a curtain or something because it's not your job to change your private environment to increase the comfort of his private environment. But then again, maybe you like your neighbors. I don't really even like the concept of neighbors close enough to be annoyed by anything I do.