You would want to set each users account password to expire sometime in the past.
From Official Ubuntu User Management Documentation:
To easily view the current status of a user account, use the following
syntax:
sudo chage -l username
The output below shows interesting facts about the user account,
namely that there are no policies applied:
Last password change : Jan 20, 2008
Password expires : never
Password inactive : never
Account expires : never
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7
To set any of these values, simply use the following syntax, and
follow the interactive prompts:
sudo chage username
The following is also an example of how you can manually change the
explicit expiration date (-E) to 01/31/2008, minimum password age (-m)
of 5 days, maximum password age (-M) of 90 days, inactivity period
(-I) of 5 days after password expiration, and a warning time period
(-W) of 14 days before password expiration.
sudo chage -E 01/31/2008 -m 5 -M 90 -I 30 -W 14 username
To verify changes, use the same syntax as mentioned previously:
sudo chage -l username
The output below shows the new policies that have been established for
the account:
Last password change : Jan 20, 2008
Password expires : Apr 19, 2008
Password inactive : May 19, 2008
Account expires : Jan 31, 2008
Minimum number of days between password change : 5
Maximum number of days between password change : 90
Number of days of warning before password expires : 14
Other Security Considerations Many applications use alternate
authentication mechanisms that can be easily overlooked by even
experienced system administrators. Therefore, it is important to
understand and control how users authenticate and gain access to
services and applications on your server.
Also, check out Automating/Customizing an Ubuntu installation using preseeding. This looks like what you want to be doing.
