How to protect users from saving password-protected DWG drawings?

You can disable Security functions during custom installation of AutoCAD.
In an already installed AutoCAD-based application, you can disable passwords using a registry setting.

Use Regedit to open the key:

HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\AutoCAD\Rxx.x \ACAD-xxxx:xxx\Applications\AcSecOpt
and set the AllowEncryption value to 0. You can change the value even when AutoCAD is running. You can force the change using Windows group policy.