Even if you have a passcode set up on your iPhone, a person could still activate Siri and use it to do any one of a number of things without permission. Thankfully this is quickly remedied with a quick change in your settings.
Start up the Settings app, tap on General, and tap on Passcode Lock. After you enter your passcode in, you’ll see this screen. All you need to do is make sure that Siri is set to “off.”
This only disables Siri while your phone is locked, it doesn’t turn it off completely.
An easy fix: