Photos for Mac cannot be deleted from an El Capitan system, because it is protected by El Capitan's System Integrity Protection. If you disable this protection and remove Photos you might run into problems because other applications may rely on its presence. And with each update it will be back.
If the problem is that you frequently need to format your cards and the fix Larry provided will not suffice, disable all photo applications from launching at all:
A brute-force method to stop any application from launching at all when you insert a card is a Terminal command:
This Terminal command will disable the auto start of any photo application, not just Photos. So Lightroom or any other application will not launch either and you will have to launch the application of your choice manually.
- 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
defaults -currentHost write com.apple.Photos 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