I would inform the bosses that if they do not stop this guy from doing this then you will take matters in your own hands...and either have him arrested or you will quit your job. You should not have to tolerate this at all in the work place.

Discuss this with your boss, since legally the desk is their property. Set up a hidden camera on the desk and then leave a large enough amount of money in the drawer. When he takes it, they can deal with him. He most likely will decide to return the money and resign rather than have the police involved. Method 2.... leave a dead animal in your drawer. Just the surprise of seeing it should be enough for him to realize that he doesn't belong there. Not only is it horrible to find, but since he wasn't supposed to be there he has no one he can really tell about it. If he does tell someone else about it, it will just start the discussion about why he was there in the first place when he doesn't have a right to be. You have personal property in your desk, you have a right to feel secure within your own office.

then you need to make more complaining noise - make it formal and put it in writing - be sure to include that should this continue you will be forced to take action to recover your property and will have no hesitation in calling the Police and reporting this is as a crime. Also add that you will be holding the company full accountable for any financial loss incurred should they fail to rectify this untenable situation. Copy the letter to immediate manager and make sure you send copies to Personnel and Senior Management (take this above your boss, go to his boss). Request they either sit him or yourself elsewhere. If all else fails, go to doctor, tell them, say you are stressed out and then have the next year off of work on full pay - at which point you can utter the words "How do you like me now". Of course, if all this is too much grief and this is starting to take over personal time and is upsetting your life outside of work, there is nothing wrong about finding another job - it is easier and sometimes, we all need 'easier'. If so, forget the morals and all that crap and what you 'should' do because it's the 'right' thing and just walk away as soon as you get something else. Hmm, a good use of the company internet link methinks.