I'm trying to delete an Address Book Policy that has been created in Office 365, but it won't allow me as it's still assigned to users. What PowerShell command can I use that will remove the Policy for all my users, or users with a specific attribute set.

The address book policy "ABP_Students" Custom Attribute 1 for these users is set to "students" to easily pull them out... also "Department" for these users is set to "students"

Many Thanks