if you laugh at me for this i'll h8 you forever
I used to live in a similar situation when I first lived away from home, and the people upstairs used to always leave the downstairs door open, jarred open all the time. We lived in a fairly bad area, too, and I was petrified one night when someone random came off the street and tried opening my door.
Soooooo
This one night when I knew they'd all gone out partying, I broke in and rearranged all their furniture.