Siri is an excellent feature on iPhone and iPad. It has been there on iDevice since the launch of iPhone 4s, and it is widely used by iPhone users, who don't want to use small keypad of the phone screen (iPad users have the benefit of a bigger keypad).
But what happens when Siri starts working automatically without receiving any command from you? It would be a fun to imagine this. When you involuntarily press the home button of you iPhone when it is locked and Siri asks: What can I help you with?
There can be many situations when you accidentally press home button; when you iPhone is in your pocket, in bag or when you hold it in your hand. This could trigger Siri and it could start some action too. If you want to turn off Siri on lockscreen in your iPhone or iPad, here is a simple method to follow.
How to Turn Off Siri on Lockscreen in iPhone/iPad on iOS 8.4
Step #1. Launch Settings app on your iPhone/iPad.
Step #2. Tap on Touch ID & Passcode.
Step #3. Enter your passcode.
Step #4. Scroll down to the section: ALLOW ACCESS WHEN LOCKED.
Step #5. Toggle the option of Siri OFF.
This action will turn off Siri on lockscreen in your iDevice on iOS 8.4. If you are one of the rare users, who use iOS 7 on your iPhone/iPad, you can also turn off Siri on a lockscreen; however, it's a different method.
Please note that once you toggle Siri OFF, the Voice Dial option above will be toggled OFF automatically.