Go back a couple of years and I was having to do it several times a week. One of the residents in a flat upstairs went down the path of alcoholism. After a while, once he'd spent all his money, he funded his drinking by allowing street drunks to sleep in his apartment. Pretty soon they were followed by the street junkies, and before you know it, the dealers had moved in and were using the flat as a crack house. Other residents on that floor were too scared to come out of their own homes, either during the day or overnight. I lost count of how many arguments I had at the entrance to the buildilng blocking people entry, and I was physically assaulted on one occasion by a couple of crackheads who snuck up behind me. Although they came off worse (a passer-by had to restrain me from literally kicking one of their heads in, which I'm not proud of, but seemed like the only thing to do at the time), the final straw came when a bunch of heavy-looking Africans turned up one night, threw a block of concrete through the building's front door, kicked the door in of the crack flat, dragged one of the dealers out screaming and bundled him into a car. In total this was a period of about eight months, in which the caretaker was constantly cleaning up discarded syringes, the stairwells were full of druggies every night, and you would be constantly awoken by either screaming, arguing, or even someone going cold turkey out in the corridor.
Anyway, finally we got some real action. The Police set up surveillance in a building opposite, arrested the dealers and sent them down. The local council helped us hire some hard-ass security guards to stand on the entrances 24 hours a day, and they arranged ASBOs against all the druggies and street drunks so that if they tried to enter the building, the Police could immediately arrest them. We then got the entrances of the building refurbished with much stronger doors.
You wouldn't believe the difference it's made. The building is now a nice place to live again, although we had to get virtually all the common areas redecorated to repair the damage. The chap who's drinking started it all sold up and moved on. I understand he's living in sheltered housing up in North London now, although whether he's still drinking I don't know.