The webcam with motion detector software is the best bet.
Cheap, disguised in plain sight and easily checked for activity. make sure you have a screen saver with passowrd activated so they can't snoop on your computer.
So, buy an inexpensive web cam and put it on top of your computer monitor.
Put some "bait" like panties, sexy pictures of yourself, or something else that they should not be touching etc in plain sight but partially covered directly in the view of the webcam.
Get the guy coming in a few times to prove that he is doing so with criminal intent and not entering your place because of an emergency (the only reason he can enter your residence without notice) .
Go to the police, the landlord (owner of the building) and your civil lawyer (in that order) with your evidence.
INSIST that the police investigate the possibility of other victims in your building and insist that he be charged.
Get your lawyer to sue the landlord also.