* password protect Excel files
* password protect Win XP (if that's your operating system)
* catch them at it, and punch them in the head: many people respond to
this type of beating
* lock the room
When you start a computer, there are many many files that are updated
with data that can be used to show that someone logged on at a
particular time. If they log on after hours after they have been told
not to do so, use this as the compelling event to fire them. Keystroke
logger software might be used to show what this person is doing on the
computer, although you'll probably find out they are goofing on IM
rather than selling company secrets. It's easy to assume malice when
the true motivation is stupidity.