You can block the application through the Restrictions menu. You need to have the WhatsApp installed so you can select it in the menu. Go to General > Restrictions and then locate the app. Turn the slider next to WhatsApp and enter a password. No one will ever be allowed to access, delete, install, etc., that app until you enter the passcode again.
For additional information, visit the Apple Support page: https://support.apple.com/en-us/HT201304
