Sounds bizarre that an apartment building of over 80 units has generic locks and keys! Never heard of such a thing! If you are the owner, I would have changed the locks when I first moved in. if you are a tenant, you will have to go through the property manager for permission to change locks. As for your neighbour….why are they coming into your unit? What are they doing there? What telltale signs are they leaving? Of course you could set up security cameras of your own inside with a link to your mobile phone to record and video illegal entry, or hire a security company to do so. Security cameras are on sale at Bunnings, so hot foot it down there……..Or set a big mousetrap!