You'll probably find that the 'Image Capture' application (Applications folder) is the culprit. I suggest you connect your card reader and memory card then launch Image capture. On bottom left corner you can set the import device to 'No application' or if you prefer 'Lightroom'. Typically, you will need to choose Lightroom by first selecting other from the drop down list. See below screenshot from Image Capture.
If you REALLY don't want Apple's Photos app then type this in Terminal:
sudo rm -rf /Applications/Photos.app
enter your password when prompted and POOF Photos is gone!
Thanks, Ian. I tried this, and it worked repeatedly as long as I was using the same SD card. When I switched to a new card, the problem started again. Apparently the system (the Image Capture app?) treats each memory card as a different device.
This is a Macbook Pro with a built-in card reader.
Thanks, Joshua, but I don't want to erase the Photos app completely; I just want it not to be the default for importing. I have many old photos in iPhoto that I've never gotten around to porting to Lightroom. I assume they will transition painlessly to Photos.
I solved this problem with the free RCDefaultApp control panel:
The “Camera” option also sets the default app for flash cards inserted via card readers. Or at least it does for me.