|If a person has access they will find out . . . .|
To be sure - keep files on another computer with back ups
If they need to be used in the 'problem' computer use a copy on external hard drive - plug the HD into a USB port only when necessary.
After everything is working correctly and backed/ imaged on both systems then delete the files on the 'problem' computer, clean up that computer to remove Recycles, Temps, ... etc.,
This action and any others will very likely leave logging entries scattered around in the 'problem' computer Registry and any monitoring software that may have been introduced without your knowledge - so depends how skilled your user is . . . .