Change Password to initial

Aloha

What is the best way to mark a user Password administratively as Initial?

I dont want to have a new Password, only the aktually Password should be Initial.

I have a Group of users (not all but sure more than 500) i want to force to alter their Password.

I try to set the "Initial" flag in the USR02 on 0,1 or 2 but that make no reason at the next log in.

Thank you and sorry for my english

Maik