|The victim, with wiring visible to the left.|
First step: figure out which breaker controls the outside lights. While I went through a similar exercise with the entire house when we got the main panel replace a few months ago, I didn't know where this specific light was. I eventually found it connected to a 20-amp circuit for the downstairs bathroom, presumably so it's GFCI-protected. Not entirely the approach I'd take, but it's just another one of those "you've got to be kidding me" moments which happens whenever I open up a wall in our house.
After turning off power, I opened the face plate and pulled out the old switch. Fortunately, there wasn't anything too surprising, and I was able to disconnect the switch from it's power source and cap both the wires leading to the outdoor light.
The light outside was a little harder to deal with, as the screws which fastened its junction box to the deck had long since been painted over. After a couple of attempts with the impact driver, I decided it was worth it to just come back later with the reciprocating saw.