Silly Tilly
We had a period of kids sitting on the wall outside our block. So to prevent it we planted thorny plants that grow all around and over walls and there has been no problems since the plants have grown.

Change your wall so that it cant be sit upon. A quick way could be that you have a metal upright cemented in along the middle of the wall (Even a spikey one).

I used to deliver leaflets in the area and I identified that passage you mentioned as high risk just by looking at it (Without any events). In fact it might be best for it to be sealed off.

After that a friend of mine was mugged there and it will appeal to muggers because it offers a dual escape route.