When the Import panel in Photos opens, you should see a checkmark in the toolbar "Open Photos for the device". Disable this checkmark and Photos should not open again for this camera (until you reformat the card).
The automatic launch can only be disabled for each camera separately.
You may want to try this fix: This Terminal command will disable the auto start of any photo application, not just Photos.
- Quit Photos and disconnect any camera.
- Open a Terminal and paste this command into it and hit return:
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool YES
Restart the Mac . it should not be necessary, but it only worked for me after restarting.
I found this fix in a post by JadedEye (to léonie)
https://discussions.apple.com/thread/7176882?answerId=29506561022#29506561022