Stop iTunes from opening when a device is plugged in?

Under System Preferences → Users → [you] → Login Items → select iTunesHelper, then click the minus sign and log out/into your Mac.

No devices will cause iTunes to launch (that's the whole purpose of iTunesHelper).

In iTunes there is an option to prevent the app to launch when a device is connected to your computer.

Open iTunes > Clic on a device in the sidebar > Scroll down to the "Options" section.

Then you have to make sure this little checkbox is un-check:
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If the problem still occurs after that, come back here !

Open iTunes -> Preferences -> Devices.

Make sure Prevent iPods, iPhones, and iPads from syncing automatically option is checked.