I am not sure if you can completely prevent the device from allowing a passcode to be set. I do know that if a passcode was set by the user as the result of a policy requiring a passcode, and you remove that policy from the device, the end user will have to manually go into settings and remove their passcode. The removal of the policy unfortunately does not remove the passcode if one is set. You then would have to do what @tsossong said and send the Clear Passcode command to all of the devices.
So with clearing the passcode, it might be the way around it and unfortunately I don't think you can scope out an action to happen at a reoccurring interval on the iPads that had a passcode set to automatically remove it.
You can however make a smart group for all the devices that have a passcode and manually send the command to Clear Passcode to all of the devices in that smart group at one time. When you 'View' the contents of the smart group, click on 'Actions' at the bottom, choose 'Send Remote Commands' from the list and 'Clear Passcode (does not apply to personally owned devices)' is one that you can send to the group. Granted they are not personally owned of course.
