You can use CCleaner to remove System Restore points (see below for details). Once you remove one, nobody will be able to restore the system to that point.
To remove System Restore points:
There are three System Restore points, as follows:
When you uninstalled Office, Windows deleted the software and related configuration files. If someone restored the system to the September 15, 2009 point, Office 2007 would also be restored.
If you delete the September 15, 2009 point, someone can still restore the system to the September 1, 2009 point to get to Office. The solution is to go back as far as the program was installed (if possible) and to remove that System Restore point.
About System Restore:
System Restore in Windows takes regular snapshots of system activity and crucial files. In the event that your programs become corrupted or that you want to roll back the system to an earlier date, you can choose a System Restore point (the 'snapshot') to restore Windows to the state it was in on that date.
While this is great for fixing software problems after the fact, it can lead to privacy issues. You may have deleted files or uninstalled programs, but if a System Restore point Windows made before that time exists, someone else can restore the files and software at a later time.
Rather than disable System Restore, you can use CCleaner to selectively remove System Restore points (see above).
Note: CCleaner removes references to the System Restore points, but may not actually remove all files related to each point.