Log into the file as sysadmin in single-user mode and change everyone's passwords to something else is one option.  What they don't know can't hurt your file.

In your live file you should also log in as sysadmin in single-user mode and turn on the setting, Setup, Settings, Company, System, Do not allow transactions before xxx and enter the first day of your new fiscal year.  This will prevent users from posting to the previous year.

Hi Richard, if I change their password to something else, will it only apply to the file I don't want them to use, and they will be able to use their old password for the live file?

And yes, I did set up Sage to not allow transactions before our year-end date.

Yes, files are independent of each other.  You can run two company files on the same computer with different passwords for the same username.

Wonderful! This ought to solve my problem then! Thank you