To remove the camera from the device in a muti-app kiosk, you can edit the kiosk policy. In the kiosk policy, please navigate to Android Kiosk Lockdown->Multi App and remove the camera app. Click Save.
You can also disable the camera app through restriction. For this, navigate to Android-> Restrictions -> Allow Device Functionality. Disable the option ‘Camera’. Please note to associate targets and save the policy.
Please use the help link for more info on restrictions in Android.