To set options for Siri, go to Settings > General > Siri. Options include:

• Turning Siri on or off
• Language
• Voice gender (may not be available in all areas)
• Voice feedback
• My Info card

Prevent access to Siri when iPad is locked. Go to Settings > General > Passcode Lock. You can also disable Siri by turning on restrictions. See the Basics section -> Restrictions.