Best Advice, is not to become too paranoid, but think of this as a wake-up call. Start by monitoring any traffic conducted from your workstation (i.e. collect detailed evidence – times, dates, places). A good poker player never gives in his hand. Above all “Do Not Let It Get Personal” most of us can’t control our anger. Another thing these infractions are not isolated, there tends to be a pattern of abuse and your other work colleagues are in most cases are aware of it too. Now a Big Question is how to tread, I believe the SLOW and methodical approach is the best option, collect that evidence in case you have to cover your A**. Once your are sure of all your evidence then seek out the authorities.
**Again I say, Do Not Let It Get Personal, but if you can not handle it make sure you have plenty witnesses when you make your confrontation. I myself am like the Lawn-mower man if I think it’s is Personal!