You can block the users from modifying the configured password on iOS devices:
Navigate to Policies-> iOS-> Advanced Restrictions-> Allow security and Privacy Settings and disable the option ‘Modify passcode’
Associate targets and save the policy.
The users will not be able to add, change or remove the device passcode. Please note that a password policy cannot be associated with the device if this option is disabled. The feature is supported only on Supervised iOS 9.0+ devices.